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/vision.c

@@ -837,6 +837,8 @@ vision_recalc(int control)
     /* Set the new min and max pointers. */
     gv.viz_rmin = next_rmin;
     gv.viz_rmax = next_rmax;
+
+    notice_all_mons(TRUE);
 }
 
 /*