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

[kevans]

Summary

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.

[kevans]

Rebase... I reparented it to the jail.list() review because that's, unfortunately, how I've worked on it locally... if it's easier to work with a git branch, I've got it published here: https://github.com/kevans91/freebsd/tree/kbsd/lua-libs

[freqlabs]

lib/flua/libjail/lua_jail.c

682

It could also be the name as a string. Likewise below for remove.

[freqlabs]

Apart from the comment nit this looks good. Tested it a bit with io.popen("ps ax") to verify we do get jailed.