Allow FIONBIO and FIOASYNC ioctls on POSIX shm descriptors.

They have no effect, as with filesystem file descriptors.
This improves compatibility with some existing userspace code.

Submitted by: Greg V <greg@unrelenting.technology>
Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19330