Server prep: routine to create a local X display.

This will be used for the server side of X forwarding. It wraps up the
mechanics of listening on the right TCP port and (if possible) the
associated AF_UNIX socket, and also creates an appropriate X authority
file containing authorisation data provided by its caller.

Like the new platform_create_agent_socket, this function spawns a
watchdog subprocess to clean up the mess afterwards, in the hope of at
least _most_ of the time not leaving old sockets and authority files
lying around /tmp,

ssh.h

@@ -1002,6 +1002,9 @@ char *platform_get_x_display(void);
  */
 void x11_get_auth_from_authfile(struct X11Display *display,
 				const char *authfilename);
+void x11_format_auth_for_authfile(
+    BinarySink *bs, SockAddr *addr, int display_no,
+    ptrlen authproto, ptrlen authdata);
 int x11_identify_auth_proto(ptrlen protoname);
 void *x11_dehexify(ptrlen hex, int *outlen);