RFC posix mqueue namespace (v11)

(Ok I don't know what the actual version number is - it's high but 11 is probably safe)

Cedric and Nadia took several approaches to making posix message queues per-namespace. I ended up mamking some deep changes so am not retaining their Signed-off-by:s on this version, but this is definately very much based on work by both of them.

Patch 2 hopefully explains my approach. Briefly,

1. sysv and posix ipc are both under CLONE_NEWIPC
2. the mqueue sb is per-ipc-namespace

So to create a new ipc namespace, you would

	unshare(CLONE_NEWIPC|CLONE_NEWNS);
	umount /dev/mqueue
	mount -t mqueue mqueue /dev/mqueue

It's perfectly valid to do vfs operations on files in another ipc_namespace's /dev/mqueue, but any use of mq_open(3) and friends will act in your own ipc_ns.

thanks,
-serge