autodescribe for #terrain

autodescribe for #terrain

Requested during beta-testing however long ago:  want a way to
look at specific map locations while #terrain is showing them
without monsters and/or objects and/or traps being displayed in
the way.  The post-3.6.0 autodescribe feature for getpos() made
this pretty easy to achieve, although the lookat() aspect felt
more like trail-and-error than careful design.

Instead of putting up a --More-- prompt, ask the player to pick
a location with the cursor.  Moving the cursor gives the terse
description for every location traversed.  Actually picking a
spot just ends #terrain and goes back to normal play.