[rbldnsd] ipv6 support: beginnings of an ip6trie dataset

[Michael Tokarev]

I looked at the whole thing again, now more seriously.  I'm sorry this
took so long, -- somehow I've been very busy almost all this time...

The new thing is just excellent.  I really really like it, especially
the way how it performs much better than my ip4trie code, -- much
faster and uses less memory.  Fun.

I did quite some tests using real-life data - current dump of cbl.abuseat.org
(xbl.spamhaus.org) data, a bit more than 9.5mil entries, the load speed
is comparable with even ip4tset (the latter is about 3 times faster but
that's expected.  My ip4trie is about 10 times slower than tset).  Very
impressive.

I made a few more cosmetical changes on top of your branch,

I pushed the resulting thing to my git repository, and this is about to
become 0.997(+ipv6) release, first release after 5 years.

Please take a look - how do you like my changes... and let's release the
damn thing :)

Please note: I haven't tested actual IPv6 code, especially on uncommon
architectures.  I probably should do that before actual release ;)

Thank you very much for your contributions, Jeff!

/mjt