Add an option to give feedback when bumping walls

Boolean option "mention_walls", defaults to off.
Useful for blind players.

src/options.c

@@ -139,6 +139,7 @@ static struct Bool_Opt
 #else
 	{"mail", (boolean *)0, TRUE, SET_IN_FILE},
 #endif
+	{"mention_walls", &iflags.mention_walls, FALSE, SET_IN_GAME},
 	/* for menu debugging only*/
 	{"menu_tab_sep", &iflags.menu_tab_sep, FALSE, SET_IN_GAME},
 	{"mouse_support", &iflags.wc_mouse_support, TRUE, DISP_IN_GAME},	/*WC*/