mdmon: wait after trying to kill

mdmon: wait after trying to kill

Now that mdmon handles sigterm if another monitor wants to take over it
should wait until all managed arrays are clean.  So make WaitClean()
available to mdmon and teach try_kill_monitor() to wait on each subarray
in the container.

...since we may be communicating with a dieing process, we need to
block SIGPIPE earlier.

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