build: New configure option --enable-run-gnupg-user-socket.

* configure.ac: (USE_RUN_GNUPG_USER_SOCKET): New ac_define.
* common/homedir.c (_gnupg_socketdir_internal): Add extra directories.

This allows to build GnuPG with an extra socketdir below /run. See
https://lists.gnupg.org/pipermail/gnupg-devel/2017-November/033250.html
for a longer explanation why this is sometimes useful.

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