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!