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.

proxy.c

@@ -368,7 +368,7 @@ static char *dns_log_msg(const char *host, int addressfamily,
 }
 
 SockAddr name_lookup(const char *host, int port, char **canonicalname,
-		     Conf *conf, int addressfamily, void *frontend,
+                     Conf *conf, int addressfamily, Frontend *frontend,
                      const char *reason)
 {
     char *logmsg;