Change key binds from array to linked list

Key bindings were stored as a fixed-size array, indexed by the input
character, pointing to the extended commands.  This changes that into
a linked list of an intermediary struct Cmd_bind, storing the input key
and the pointer to the command.

This is just code cleanup for future enhancements, and should have
no effect on gameplay.

src/options.c

@@ -7682,7 +7682,7 @@ parsebindings(char *bindings)
     }
 
     /* extended command? */
-    if (!bind_key(key, bind)) {
+    if (!bind_key(key, bind, TRUE)) {
         config_error_add("Unknown key binding command '%s'", bind);
         return FALSE;
     }