ctdb-cluster-mutex: Block signals around fork

ctdb-cluster-mutex: Block signals around fork

If SIGTERM is received and the tevent signal handler setup in the
recovery daemon is still enabled then the signal is handled and a
corresponding event is queued.  The child never runs an event loop so
the signal is effectively ignored.

Resetting the SIGTERM handler isn't enough.  A signal can arrive
before that.

Block SIGTERM before forking and then immediately unblock it in the
parent.

In the child, unblock SIGTERM after the signal handler is reset.  An
explicit unblock is needed because according to sigprocmask(2) "the
signal mask is preserved across execve(2)".

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13617

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>