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/uxser.c

@@ -544,7 +544,7 @@ static int serial_ldisc(void *handle, int option)
     return 0;
 }
 
-static void serial_provide_ldisc(void *handle, void *ldisc)
+static void serial_provide_ldisc(void *handle, Ldisc *ldisc)
 {
     /* This is a stub. */
 }