If you configure Unix PuTTY to use a proxy, tell it to even proxy

localhost connections, and also enable X forwarding in such a way that
it will attempt to connect to a Unix-domain X server socket, an
assertion will fail when proxy_for_destination() tries to call
sk_getaddr(). Fix by ensuring that Unix-domain sockets are _never_
proxied, since they fundamentally can't be.

[originally from svn r9688]

unix/uxnet.c

@@ -390,6 +390,11 @@ int sk_address_is_local(SockAddr addr)
     }
 }
 
+int sk_address_is_special_local(SockAddr addr)
+{
+    return addr->superfamily == UNIX;
+}
+
 int sk_addrtype(SockAddr addr)
 {
     SockAddrStep step;