IP address class

I've drafted an interface for a new class that wraps both IPv4 and
IPv6 addresses (plus another one for IP prefixes). It's committed to
src/IPAddr.h in branch topic/v6-addr.

This is completely untested and doesn't have an implementation yet (no
compiler has even looked at the code yet :-). For now, it's really
just to nail down the interface.

I looked at quite a bit of the "ifdef BROv6" and I believe this should
generally work as a replacement (with a bit more tweaking here and
there I'm sure).

Jon, do you think this is something you could use as starting point to
begin removing the configure switch and replace current IP address
code with uses of this wrapper?