Add new file handle system calls.
Namely, getfhat(2), fhlink(2), fhlinkat(2), fhreadlink(2). The
syscalls are provided for a NFS userspace server (nfs-ganesha).
Submitted by: Jack Halford <jack@gandi.net>
Sponsored by: Gandi.net
Tested by: pho
Feedback from: brooks, markj
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D18359