agent: Move inotify code to common and improve it.

* common/sysutils.c: Include sys/inotify.h.
(my_error_from_syserror, my_error): New.
(gnupg_inotify_watch_socket): New.
(gnupg_inotify_has_name): New.
* agent/gpg-agent.c: Do not include sys/inotify.h.
(my_inotify_is_name): Remove.
(handle_connections): Remove HAVE_INOTIFY_INIT protected code and use
the new functions.

When removing not a simple socket file but the entire directory the
old code missed most events and thus did not worked properly.

IN_DELETE_SELF has also been added to the watch list to detect a
removal of the directory. However, in all tests that event was not
triggered. The only way it could be triggered was by not watching
the socket dir but an arbitary directory and rmdir that.

• GnuPG-bug-id: T2756
• Signed-off-by: Werner Koch <wk@gnupg.org>