Add fchroot(2)

This is similar to chroot(2), but takes a file descriptor instead
of path. Same syscall exists in NetBSD and Solaris. It is part of a larger
patch to make absolute pathnames usable in Capsicum mode, but should
be useful in other contexts too.

Reviewed By: brooks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41564