Turned the old `Telnet Command' System-submenu into a more general

`Special Command' menu, in which any backend can place its own list
of magical things the user might want to ask the backend to do. In
particular I've implemented the recently proposed "break" extension
in SSH2 using this mechanism.
NB this checkin slightly breaks the Mac build, since it needs to
provide at least a stub form of update_specials_menu().

[originally from svn r3054]

unix/pterm.c

@@ -148,6 +148,14 @@ void ldisc_update(void *frontend, int echo, int edit)
      */
 }
 
+void update_specials_menu(void *frontend)
+{
+    /*
+     * When I implement a context menu in pterm, I will need to
+     * support this function properly.
+     */
+}
+
 int askappend(void *frontend, Filename filename)
 {
     /*