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.

src/monmove.c

@@ -1307,6 +1307,8 @@ postmov(
     boolean canseeit = cansee(mtmp->mx, mtmp->my),
             didseeit = canseeit;
 
+    notice_mon(mtmp);
+
     if (mmoved == MMOVE_MOVED) {
         nix = mtmp->mx, niy = mtmp->my;
         /* sequencing issue:  when monster movement decides that a