Add shm_create_largepage(3) helper for creation and configuration of
largepage shm objects.

And since we can, add memfd_create(MFD_HUGETLB) support, hopefully
close enough to the Linux feature.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D24652