Don't bind quit to any key by default

Quit is not a commmand you usually need very often, and generally
don't want to use by accident.

Apparently, on Spanish keyboard layout, n-with-tilde is interpreted
by the Windows NetHack as M-q, and the key is next to l. Loot also
asks for confirmation, just like quit.

Prevent stuff like this by not binding the quit command to any key.

src/cmd.c

@@ -1908,7 +1908,7 @@ struct ext_func_tab extcmdlist[] = {
               doputon, 0, NULL },
     { 'q',    "quaff", "quaff (drink) something",
               dodrink, 0, NULL },
-    { M('q'), "quit", "exit without saving current game",
+    { '\0', "quit", "exit without saving current game",
               done2, IFBURIED | AUTOCOMPLETE | GENERALCMD, NULL },
     { 'Q',    "quiver", "select ammunition for quiver",
               dowieldquiver, 0, NULL },