MFC r290913,r291181:

r290913:

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

Sponsored by: EMC / Isilon Storage Division

r291181:

Integrate contrib/netbsd-tests/kernel/t_mqueue into the FreeBSD test
suite as tests/sys/kern/mqueue_test