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/window.c

@@ -542,9 +542,20 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 	}
 
 	/*
-	 * Trim a colon suffix off the hostname if it's there.
+	 * Trim a colon suffix off the hostname if it's there. In
+	 * order to protect IPv6 address literals against this
+	 * treatment, we do not do this if there's _more_ than one
+	 * colon.
 	 */
-	cfg.host[strcspn(cfg.host, ":")] = '\0';
+	{
+	    char *c = strchr(cfg.host, ':');
+
+	    if (c) {
+		char *d = strchr(c+1, ':');
+		if (!d)
+		    *c = '\0';
+	    }
+	}
 
 	/*
 	 * Remove any remaining whitespace from the hostname.