lockf: allow locking file descriptors

This is most useful inside a shell script, allowing one to lock just
portions of a script rather than having to wrap the entire script in a
lock.

PR: 262738
Reviewed by: 0mp, allanjude (both previous versions)
Co-authored-by: Daniel O'Connor <darius@dons.net.au>
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D42718