qemu-nbd: Honor SIGINT and SIGHUP

Honoring just SIGTERM on Linux is too weak; we also want to handle
other common signals, and do so even on BSD.  Why?  Because at least
'qemu-nbd -B bitmap' needs a chance to clean up the in-use bit on
bitmaps when the server is shut down via a signal.

See also: http://bugzilla.redhat.com/1883608



Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200930121105.667049-2-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
[eblake: apply comment tweak suggested by Vladimir; fix ifdef around
termsig_handler]
Signed-off-by: Eric Blake <eblake@redhat.com>