Pull up following revision(s) (requested by rmind in ticket #779):
sys/kern/sys_mqueue.c: revision 1.18
- Slightly rework the way permissions are checked. Neither mq_receive() not
  mq_send() should fail due to permissions.  Noted by Stathis Kamperis!
- Check for empty message queue name (POSIX does not allow this for regular
  files, and it's weird), check for DTYPE_MQUEUE, fix permission check in
  mq_unlink(), clean up.

(snj)