Accessibility: Show a message when monster is spotted

Adds a new boolean option, spot_monsters.  If on, every time
the hero notices a monster which was out of sight before,
a message is given.  Combine with accessiblemsg to get the
monster location:

(3north): You see a newt.

Breaks saves and bones.

include/optlist.h

@@ -654,6 +654,9 @@ static int optfn_##a(int, int, boolean, char *, char *);
     NHOPTB(sparkle, Map, 0, opt_out, set_in_game,
            On, Yes, No, No, NoAlias, &flags.sparkle, Term_False,
            "display sparkly effect when resisting magic")
+    NHOPTB(spot_monsters, Advanced, 0, opt_in, set_in_game,
+           Off, Yes, No, No, NoAlias, &a11y.mon_notices, Term_False,
+           "message when hero spots a monster")
     NHOPTB(splash_screen, Advanced, 0, opt_out, set_in_config,
            On, Yes, No, No, NoAlias, &iflags.wc_splash_screen, Term_False,
            (char *)0)