Allow ip4/ip6 inherit

[cqexbesd]

Allow users to set ip4 or ip6 to inherit.

This isn't perfect as:

1. there isn't support for creating these jails in the first place (I create one close to what I want and edit jail.conf by hand)
2. there is an assumption in Bastille that jail will have an IP, and that pf will be in use so things like stop, while working, also emit Jail IP not found: nnnn from rdr.sh

However it seems to work and might satisfy #369 and #405.

I tried to address the stop IP assumption but I wasn't sure enough about the circumstances when there will be pf rules in place. Perhaps someone who understands all he permutations could produce a table of the different allowed values of ip[46]* and what it means for other networking things (pf rules, vnet interfaces, pairs etc). Then there could be a common set of functions other scripts could call to check if they should be doing any of that stuff.

NB this PR assumes #534 gets pulled first - then this becomes just the one commit.

[arrowd]

I found it disappointing that it is not possible to just inherit ip4 and ip6 from the host. Looking forward for this PR get merged in.

[bmac2]

534 has been previously merged.

[bmac2]

@cqexbesd I want to merge this but need you to update the code to resolve the conflicts. Sorry it took us this long to get to this.

[cqexbesd]

Rebased

[bmac2]

will review with @cedwards this week and get merged.

[cedwards]

I like this but let's work on expanding it to full support before we merge anything. In other words, let's add support for creating jails using inherit and make it a first class citizen.

For my own reference, I would appreciate any real examples of using inherit (honestly not a feature I ever personally use, hence the lack of current support). Also, any docs or code on how iocage or others handle inherit-based jails would be nice for me to better understand.