copy messages directly from sender into a receiver-supplied buffer

Require the receiver to pre-allocate a buffer and register it with
HELLO. The sender will copy its data directly into the revceiver's
buffer.

The RECV call will now only return a pointer to the next message
in the buffer. The buffer needs to be free()d with the FREE ioctl.

FIXME: currently missing is a proper allocator to manage the
receiver's buffer, in the current state the allocator will only clear
the buffer if all messages are free()d. This is obviously only useful
for testing.
rename files: ns -> namespace, ep -> endpoint
message: cgroup_lock is private, we can live without it for our testing for now.