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.

doc/Guidebook.mn

@@ -1983,6 +1983,13 @@ compile time option AUTOPICKUP_EXCEPTIONS included, you may be able to use
 configuration file lines to further refine
 .op autopickup
 behavior.
+.lp pile_limit
+When walking across a pile of objects on the floor, threshold at which
+the message "there are few/several/many objects here" is given instead
+of showing a popup list of those objects.  A value of 0 means "no limit"
+(always list the objects); a value of 1 effectively means "never show
+the objects" since the pile size will always be at least that big;
+default value is 5.
 .lp playmode
 Values are `normal', `explore', or `debug'.
 Allows selection of explore mode (also known as discovery mode) or debug