sd-journal: add an API to enumerate known field names of the journal

This adds two new calls to get the list of all journal fields names currently in use.

This is the low-level support to implement the feature requested in #2176 in a more optimized way.

diff --git a/man/sd_journal_enumerate_fields.xml b/man/sd_journal_enumerate_fields.xml
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+++ b/man/sd_journal_enumerate_fields.xml
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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  This file is part of elogind.
+
+  Copyright 2016 Lennart Poettering
+
+  elogind is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  elogind is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with elogind; If not, see <http://www.gnu.org/licenses/>.
+-->
+
+<refentry id="sd_journal_enumerate_fields">
+
+  <refentryinfo>
+    <title>sd_journal_enumerate_fields</title>
+    <productname>elogind</productname>
+
+    <authorgroup>
+      <author>
+        <contrib>Developer</contrib>
+        <firstname>Lennart</firstname>
+        <surname>Poettering</surname>
+        <email>lennart@poettering.net</email>
+      </author>
+    </authorgroup>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>sd_journal_enumerate_fields</refentrytitle>
+    <manvolnum>3</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>sd_journal_enumerate_fields</refname>
+    <refname>sd_journal_restart_fields</refname>
+    <refname>SD_JOURNAL_FOREACH_FIELD</refname>
+    <refpurpose>Read used field names from the journal</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <funcsynopsis>
+      <funcsynopsisinfo>#include &lt;elogind/sd-journal.h&gt;</funcsynopsisinfo>
+
+      <funcprototype>
+        <funcdef>int <function>sd_journal_enumerate_fields</function></funcdef>
+        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+        <paramdef>const char **<parameter>field</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>void <function>sd_journal_restart_fields</function></funcdef>
+        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef><function>SD_JOURNAL_FOREACH_FIELD</function></funcdef>
+        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+        <paramdef>const char *<parameter>field</parameter></paramdef>
+      </funcprototype>
+
+    </funcsynopsis>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><function>sd_journal_enumerate_fields()</function> may be used to iterate through all field names used in the
+    opened journal files. On each invocation the next field name is returned. The order of the returned field names is
+    not defined. It takes two arguments: the journal context object, plus a pointer to a constant string pointer where
+    the field name is stored in. The returned data is in a read-only memory map and is only valid until the next
+    invocation of <function>sd_journal_enumerate_fields()</function>. Note that this call is subject to the data field
+    size threshold as controlled by <function>sd_journal_set_data_threshold()</function>.</para>
+
+    <para><function>sd_journal_restart_fields()</function> resets the field name enumeration index to the beginning of
+    the list. The next invocation of <function>sd_journal_enumerate_fields()</function> will return the first field
+    name again.</para>
+
+    <para>The <function>SD_JOURNAL_FOREACH_FIELD()</function> macro may be used as a handy wrapper around
+    <function>sd_journal_restart_fields()</function> and <function>sd_journal_enumerate_fields()</function>.</para>
+
+    <para>These functions currently are not influenced by matches set with <function>sd_journal_add_match()</function>
+    but this might change in a later version of this software.</para>
+
+    <para>To retrieve the possible values a specific field can take use
+    <citerefentry><refentrytitle>sd_journal_query_unique</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Return Value</title>
+
+    <para><function>sd_journal_enumerate_fields()</function> returns a
+    positive integer if the next field name has been read, 0 when no
+    more field names are known, or a negative errno-style error code.
+    <function>sd_journal_restart_fields()</function> returns
+    nothing.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Notes</title>
+
+    <para>The <function>sd_journal_enumerate_fields()</function> and <function>sd_journal_restart_fields()</function>
+    interfaces are available as a shared library, which can be compiled and linked to with the
+    <constant>libelogind</constant> <citerefentry
+    project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry> file.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Examples</title>
+
+    <para>Use the <function>SD_JOURNAL_FOREACH_FIELD</function> macro to iterate through all field names in use in the
+    current journal.</para>
+
+    <programlisting>#include &lt;stdio.h&gt;
+#include &lt;string.h&gt;
+#include &lt;elogind/sd-journal.h&gt;
+
+int main(int argc, char *argv[]) {
+        sd_journal *j;
+        const char *field;
+        int r;
+
+        r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
+        if (r &lt; 0) {
+                fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
+                return 1;
+        }
+        SD_JOURNAL_FOREACH_FIELD(j, field)
+                printf("%s\n", field);
+        sd_journal_close(j);
+        return 0;
+}</programlisting>
+
+  </refsect1>
+
+  <refsect1>
+    <title>See Also</title>
+
+    <para>
+      <citerefentry><refentrytitle>elogind</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>elogind.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_journal_query_unique</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_journal_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>