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.

defs.h

@@ -49,6 +49,8 @@ typedef struct Backend_vtable Backend_vtable;
 typedef struct Ldisc_tag Ldisc;
 typedef struct LogContext_tag LogContext;
 
+typedef struct Frontend Frontend;
+
 typedef struct ssh_tag *Ssh;
 
 /* Note indirection: for historical reasons (it used to be closer to