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.

[originally from svn r5047]
[this svn revision also touched putty-wishlist]

Recipe

@@ -72,6 +72,11 @@
 #      build, since at the time of writing this <multimon.h> is
 #      known not to be available in Cygwin.
 #
+#  - COMPAT=/DNO_IPV6
+#      Disables PuTTY's ability to make IPv6 connections, enabling
+#      it to compile under development environments which do not
+#      support IPv6 in their header files.
+#
 #  - COMPAT=/DMSVC4
 #  - RCFL=/DMSVC4
 #      Makes a couple of minor changes so that PuTTY compiles using