dirmngr: Fix for --disable-libdns usage.

* dirmngr/dns-stuff.c (enable_recursive_resolver, set_dns_nameserver)
(reload_dns_stuff): Conditionalize with USE_LIBDNS.
(get_h_errno_as_gpg_error): Map HOST_NOT_FOUND to GPG_ERR_NO_NAME.

--

get_dns_srv assumes error code of GPG_ERR_NO_NAME when no SRV record
available.

Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>
GnuPG-bug-id: 2889
(cherry picked from commit d26c51825e2255fe58305cbc1cd74fa43f80d93e)

Gbp-Pq: Name 0013-dirmngr-Fix-for-disable-libdns-usage.patch