pile_limit option - movement feedback "there are objects here" (trunk only)

Something that pops up in the newsgroup periodically, with <Someone>
inevitably pointing out the bit of code that the user needs to tweak,
about control of feedback when hero is walking across floor objects.
Implement new option ``pile_limit'' which allows user to set the point
at which the game switches from listing the objects to giving "there are
several/many objects here".  Default is 5, same as previous hard-coded
value (1 object gets listed via pline, 2..4 are listed in a corner popup,
5 or more objects yields a pline message instead).  Setting pile_limit
to 0 means no limit, so objects will always be listed regardless of pile
size.  Setting it to 1 effectively forces no listing since any non-empty
pile size is always at least that big, so can produce "there is an object
here" even though that's no briefer than a pline() to show one object.

dat/opthelp

@@ -118,6 +118,9 @@ pickup_burden when you pick up an item that exceeds this encumberance
               or overLoaded), you will be asked if you want to continue. [S]
 pickup_types  a list of default symbols for kinds of objects to autopickup
               when that option is on  [all]
+pile_limit    for feedback when walking across floor objects, threshold at
+              which "there are many objects here" is displayed instead of
+              listing the objects.  (0 means "always list objects.")  [5]
 runmode       controls how often the map window is updated when performing
               multi-step movement (various running modes or travel command):
               teleport -- don't update map until movement stops;