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/monst.h

@@ -160,6 +160,8 @@ struct monst {
     Bitfield(mtemplit, 1);  /* temporarily seen; only valid during bhit() */
     Bitfield(meverseen, 1); /* mon has been seen at some point */
 
+    Bitfield(mspotted, 1);  /* mon is currently seen by hero */
+
 #define MAX_NUM_WORMS 32    /* should be 2^(wormno bitfield size) */
 
     unsigned long mstrategy; /* for monsters with mflag3: current strategy */