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]

windows/winplink.c

@@ -231,6 +231,7 @@ static void usage(void)
     printf("  -A -a     enable / disable agent forwarding\n");
     printf("  -t -T     enable / disable pty allocation\n");
     printf("  -1 -2     force use of particular protocol version\n");
+    printf("  -4 -6     force use of IPv4 or IPv6\n");
     printf("  -C        enable compression\n");
     printf("  -i key    private key file for authentication\n");
     printf("  -s        remote command is an SSH subsystem (SSH-2 only)\n");