Add a request_keys() function with a midend wrapper.

This function gives the front end a way to find out what keys the back
end requires; and as such it is mostly useful for ports without a
keyboard. It is based on changes originally found in Chris Boyle's
Android port, though some modifications were needed to make it more
flexible.

net.c

@@ -3236,6 +3236,7 @@ const struct game thegame = {
     free_ui,
     encode_ui,
     decode_ui,
+    NULL, /* game_request_keys */
     game_changed_state,
     interpret_move,
     execute_move,