agent,dirmngr: Shutdown fix for supervised mode.

* agent/gpg-agent.c (handle_connections): Break if supervised.
* dirmngr/dirmngr.c (is_supervised): New.
(handle_connections): Break if supervised.

For supervised agent/dirmngr, 'systemctl stop' behaves just like
'gpgconf --kill', ignoring existing connections.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>