Integrate unfix.org's IPv6 patches up to level 10, with rather a lot

Integrate unfix.org's IPv6 patches up to level 10, with rather a lot
of polishing to bring them to what I think should in principle be
release quality. Unlike the unfix.org patches themselves, this
checkin enables IPv6 by default; if you want to leave it out, you
have to build with COMPAT=-DNO_IPV6.

I have tested that this compiles on Visual C 7 (so the nightlies
_should_ acquire IPv6 support without missing a beat), but since I
don't have IPv6 set up myself I haven't actually tested that it
_works_. It still seems to make correct IPv4 connections, but that's
all I've been able to verify for myself. Further testing is needed.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5047 cda61777-01e9-0310-a592-d414129be87e