Port contrib/netbsd-tests/kernel/t_mqueue.c to FreeBSD

- Add missing headers
- Ensure mqueuefs is loaded
- Make sure the mqueuefs path is absolute and relative to /
- Cast the result of mq_open returning -1 to (mqd_t) to mute a compiler
warning

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division