expanded #terrain command (trunk only; 2nd try...)

expanded #terrain command (trunk only; 2nd try...)

     Change the post-3.4.3 extended command "#terrain" so that it can be
used in normal play rather than just in wizard mode.  It's inspired by
a command in 'crawl' that lets you view the bare map without monsters,
objects, and traps so that you can see the floor at locations which have
been covered up by those things.

normal play
      redraw map to show the known portion of it without displaying
   monsters, objects, or traps; after player responds to --More--, the
   map returns to normal.

explore mode
      put up a menu so player can choose between the known portion of
   the map as above or the full map.  If the level isn't fully explored
   then the latter provides information to the player that he hasn't
   earned yet, but the _hero_ doesn't learn anything and after --More--
   the map reverts to what it showed before.  (In other words, unlike
   with magic mapping, the unknown portion doesn't become known.)

wizard mode
      put up a menu so player can choose among four alternatives:  the
   two above, the text representation of the map's internal levl[][].typ
   codes, or a legend explaining those codes.  (Originally, I wanted to
   be able to toggle back and forth between these last two, but looking
   at one and dismissing it, then reissuing #terrain to look at the
   other is much simpler to implement and is good enough.)