Expose the Ldisc structure tag throughout the code.

That's one fewer anonymous 'void *' which might be accidentally
confused with some other pointer type if I misremember the order of
function arguments.

While I'm here, I've made its pointer-nature explicit - that is,
'Ldisc' is now a typedef for the structure type itself rather than a
pointer to it. A stylistic change only, but it feels more natural to
me these days for a thing you're going to eventually pass to a 'free'
function.

unix/gtkwin.c

@@ -158,7 +158,7 @@ struct gui_data {
     char *wintitle;
     char *icontitle;
     int master_fd, master_func_id;
-    void *ldisc;
+    Ldisc *ldisc;
     Backend *back;
     void *backhandle;
     Terminal *term;