jib, jng: Move to usr.sbin and add manuals

[dteske]

Summary

Install with the jail tools instead of copying from examples.
Document topology and MAC derivation.

Relnotes: yes

[jlduran]

I'm OK with "promoting" these example scripts into actual commands. However, since this is a user-facing change for Jails, I would suggest setting Relnotes: yes, as some users already install jib,jng into /usr/local/sbin.
I would also like to get some feedback from people using jails to do some interesting things, rather than just isolating/forwarding a few services (like I do).

[kfv]

Nice move. Thanks!

[dteske]

@jlduran (and others), I've sent a call-for-review (CFR) to freebsd-jail@ ML -- if no response over the next 4 weeks, I'll probably merge without additional feedback

[jamie]

It makes sense for these to be official, though I don't have any particular feedback to offer on them (I've never actually run vnet jails myself).