Pass an Interactor to platform_new_connection.

This will mean that platform-specific proxy types will also be able to
set themselves up as child Interactors and prompt the user
interactively for passwords and the like.

NFC: nothing uses the new parameter yet.

unix/local-proxy.c

@@ -17,7 +17,7 @@
 Socket *platform_new_connection(SockAddr *addr, const char *hostname,
                                 int port, bool privport,
                                 bool oobinline, bool nodelay, bool keepalive,
-                                Plug *plug, Conf *conf)
+                                Plug *plug, Conf *conf, Interactor *itr)
 {
     char *cmd;