mdmon: terminate clean

mdmon: terminate clean

We generally don't want mdmon to be terminated, but if a SIGTERM gets
through try to leave the monitored arrays in a clean state, block
attempts to mark the array dirty, and stop servicing the socket.

When we are killed by sigterm don't remove the pidfile let that be
cleaned up by the next monitor.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>