librt/mq_getfd_np.3: Initial manual page
The mq_getfd_np function appeared in FreeBSD 11 with no documentation.
This function dereferences the mqd_t as a pointer to an int.
Relnotes: yes
MFC after: 3 days
Reviewed by: kib (previous), markj, ziaee
Differential Revision: https://reviews.freebsd.org/D43947