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