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.

pscp.c

@@ -60,7 +60,7 @@ const char *const appname = "PSCP";
  */
 #define MAX_SCP_BUFSIZE 16384
 
-void ldisc_echoedit_update(void *handle) { }
+void ldisc_echoedit_update(Ldisc *ldisc) { }
 
 static void tell_char(FILE *stream, char c)
 {