systemctl: drop hardcoded chkconfig invocation

Introduce /usr/lib/systemd/systemd-sysv-install [--root=] <action> <name>
abstraction, replacing the direct calling of chkconfig. This allows
distributions to call their specific tools like update-rc.d without patching
systemd.

Ship systemd-sysv-install.SKELETON as an example for packagers how to implement
this.

Drop the --enable-chkconfig configure option.

Document this in README and point to it in NEWS.

diff --git a/src/systemctl/systemd-sysv-install.SKELETON b/src/systemctl/systemd-sysv-install.SKELETON
new file mode 100755 (executable)
index 0000000..a53a3e6
--- /dev/null
+++ b/src/systemctl/systemd-sysv-install.SKELETON
@@ -0,0 +1,47 @@
+#!/bin/sh
+# This script is called by "systemctl enable/disable" when the given unit is a
+# SysV init.d script. It needs to call the distribution's mechanism for
+# enabling/disabling those, such as chkconfig, update-rc.d, or similar. This
+# can optionally take a --root argument for enabling a SysV init script
+# in a chroot or similar.
+set -e
+
+usage() {
+    echo "Usage: $0 [--root=path] enable|disable|is-enabled <sysv script name>" >&2
+    exit 1
+}
+
+# parse options
+eval set -- "$(getopt -o r: --long root: -- "$@")"
+while true; do
+    case "$1" in
+        -r|--root)
+            ROOT="$2"
+            shift 2 ;;
+        --) shift ; break ;;
+        *) usage ;;
+    esac
+done
+
+NAME="$2"
+[ -n "$NAME" ] || usage
+
+case "$1" in
+    enable)
+        # call the command to enable SysV init script $NAME here
+        # (consider optional $ROOT)
+        echo "IMPLEMENT ME: enabling SysV init.d script $NAME"
+        ;;
+    disable)
+        # call the command to disable SysV init script $NAME here
+        # (consider optional $ROOT)
+        echo "IMPLEMENT ME: disabling SysV init.d script $NAME"
+        ;;
+    is-enabled)
+        # exit with 0 if $NAME is enabled, non-zero if it is disabled
+        # (consider optional $ROOT)
+        echo "IMPLEMENT ME: checking SysV init.d script $NAME"
+        ;;
+    *)
+        usage ;;
+esac