Introduce a typedef for frontend handles.

This is another major source of unexplained 'void *' parameters
throughout the code.

In particular, the currently unused testback.c actually gave the wrong
pointer type to its internal store of the frontend handle - it cast
the input void * to a Terminal *, from which it got implicitly cast
back again when calling from_backend, and nobody noticed. Now it uses
the right type internally as well as externally.

ldisc.c

@@ -78,7 +78,7 @@ static void bsb(Ldisc *ldisc, int n)
 #define KCTRL(x) ((x^'@') | 0x100)
 
 Ldisc *ldisc_create(Conf *conf, Terminal *term,
-                   Backend *backend, void *frontend)
+                    Backend *backend, Frontend *frontend)
 {
     Ldisc *ldisc = snew(Ldisc);