Add the ability to close sessions. This adds *_free() functions to most

areas of the code.  Not all back-ends have been tested, but Telnet and SSH
behave reasonably.

Incidentally, almost all of this patch was written through Mac PuTTY,
admittedly over a Telnet connection.

[originally from svn r2615]

x11fwd.c

@@ -112,6 +112,12 @@ void *x11_invent_auth(char *proto, int protomaxlen,
     return auth;
 }
 
+void x11_free_auth(void *auth)
+{
+
+    sfree(auth);
+}
+
 /*
  * Fetch the real auth data for a given display string, and store
  * it in an X11Auth structure. Returns NULL on success, or an error