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.

undead.c

@@ -1314,6 +1314,26 @@ void num2grid(int num, int width, int height, int *x, int *y) {
     return;
 }
 
+static key_label *game_request_keys(const game_params *params, int *nkeys)
+{
+    key_label *keys = snewn(4, key_label);
+    *nkeys = 4;
+
+    keys[0].button = 'G';
+    keys[0].label = dupstr("Ghost");
+
+    keys[1].button = 'V';
+    keys[1].label = dupstr("Vampire");
+
+    keys[2].button = 'Z';
+    keys[2].label = dupstr("Zombie");
+
+    keys[3].button = '\b';
+    keys[3].label = NULL;
+
+    return keys;
+}
+
 static game_state *new_game(midend *me, const game_params *params,
                             const char *desc)
 {
@@ -2716,6 +2736,7 @@ const struct game thegame = {
     free_ui,
     encode_ui,
     decode_ui,
+    game_request_keys,
     game_changed_state,
     interpret_move,
     execute_move,