From: Manfred Spraul <manfred@colorfullife.com>

Make the posix message queue mountable by the user.  This replaces ipcs and
ipcrm for posix message queue: The admin can check which queues exist with ls
and remove stale queues with rm.

I'd like a final confirmation from Ulrich that our SIGEV_THREAD approach is
the right thing(tm): He's aware of the design and didn't object, but I think
he hasn't seen the final API yet.