Corosync signal handler (SIGINT and SIGTERM) is replaced by cman one,
and this was settting quit_threads to 1. Regular cman shutdown sequence
(cman_tool leave) tests if quit_threads is set or not. If so, it refuses
continue so it was not possible to cleanly leave cluster.
Now SIGINT and SIGTERM are ignored, and (un)intentional kill of corosync
is no longer problem.