Refactor the construction of X protocol greetings.

I've moved it out into a separate function, preparatory to calling it
from somewhere completely different in changes to come. Also, we now
retain the peer address sent from the SSH server in string form,
rather than translating it immediately into a numeric IP address, so
that its original form will be available later to pass on elsewhere.

[originally from svn r10080]

ssh.h

@@ -401,6 +401,10 @@ struct X11FakeAuth {
      */
     struct X11Display *disp;
 };
+void *x11_make_greeting(int endian, int protomajor, int protominor,
+                        int auth_proto, const void *auth_data, int auth_len,
+                        const char *peer_ip, int peer_port,
+                        int *outlen);
 int x11_authcmp(void *av, void *bv); /* for putting X11FakeAuth in a tree234 */
 /*
  * x11_setup_display() parses the display variable and fills in an