common: Implicitly do a gpgconf --create-socketdir.

* common/homedir.c (_gnupg_socketdir_internal): Create the
sub-directory.

Although there is no auto cleanup (yet) this should be helpful. Let's
see whether possibly leaving stale directories around is better than
running into trouble when --create-socketdir was not used.

• Signed-off-by: Werner Koch <wk@gnupg.org>