Quit with an error status on SIGTERM during migration

This prevents the supervisor from thinking that the migration completed
successfully.

In order to do this, I've introduced a new lock around the start (and
finish) of the migration so that we avoid a race between the signal
handler in the server_accept loop and the control thread mirror startup.
Without that, we'd risk successfully starting a migration after the
SIGTERM handler fired, which would be Bad.

src/mirror.c

@@ -568,6 +568,7 @@ void * mirror_super_runner( void * serve_uncast )
 
 	serve->mirror = NULL;
 	serve->mirror_super = NULL;
+	server_allow_mirror_start( serve );
 
 	mirror_super_destroy( super );
 	debug( "Mirror supervisor done." );