agent: Shutdown on removal of the home directory.

* common/sysutils.c (gnupg_inotify_watch_delete_self): New.
* agent/gpg-agent.c (handle_connections): Rename my_inotify_fd to
sock_inotify_fd.
(handle_connections): Add home_inotify_fd to watch the home directory.

Note that we should add this also to dirmngr. And for non-Linux
systems a stat in ticker should be implemented.

• GnuPG-bug-id: T3218
• Signed-off-by: Werner Koch <wk@gnupg.org>