Remove $FreeBSD$: two-line nroff pattern

Remove /^\.\\"\n\.\\"\s*\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

jail(3lua): add jail.attach()/jail.remove() methods

These aren't a part of or use libjail(3), but rather are direct
syscalls. Still, they seem like good additions, allowing us to attach
to already-running jails.

Reviewed by: freqlabs
Differential Revision: https://reviews.freebsd.org/D26927

jail(3lua): add a jail.list() method

This is implemented as an iterator, reusing parts of the earlier logic
to populate jailparams from a passed in table.

The user may request any number of parameters to pull in while we're
searching, but we'll force jid and name to appear at a minimum.

Reviewed by: freqlabs
Differential Revision: https://reviews.freebsd.org/D26756

flua: Add a libjail module

libjail is pretty small, so it makes for a good proof of concept demonstrating
how a system library can be wrapped to create a loadable Lua module for flua.

* Introduce 3lua section for man pages
* Add libjail module

Reviewed by: kevans, manpages
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D26080