Fix jail examples in jib, jng, README

The provided example jail configs do not work for multiple interfaces.
Multiple interfaces need to be specified as a comma separated list or
using multiple += lines in jail.conf. In the given example, a space-
separated string is used, which doesn't work with multiple interfaces.

Also added a note to the README about VIMAGE being built-in by default
on amd64 in FreeBSD 12, with appropriate instructions for loading the
necessary netgraph ether module (ng_ether) since it is neither built-
in nor autoloads.

Submitted by: Ryan Moeller <ryan@freqlabs.com>
Reported by: Ryan Moeller <ryan@freqlabs.com>
MFC after: 3 days
Sponsored by: Smule, Inc.
Differential Revision: https://reviews.freebsd.org/D17697