agent: Close a dialog cleanly when gpg/ssh is killed for CONFIRM.

* agent/call-pinentry.c (watch_sock_start): Factor out
from do_getpin.
(watch_sock_end): Likewise.
(do_getpin): Use those functions.
(agent_get_confirmation): Likewise.
(popup_message_thread): Likewise.

Pinentry's dialog for confirmation should be also closed cleanly, as
well as the dialog for pin-input.

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