TTY: Hilight piles of objects

Add MG_OBJPILE flag, which windowports can use to check if a location
has more than one object stack. If use_inverse is on, TTY will use
inverse to show such piles.  If a boulder is the topmost item on a pile,
then the object pile flag is not used; mainly because boulders are "solid",
boulders dropped by monsters are nearly always over other objects, and so
that special levels such a Sokoban can "hide" items under the boulders.

TODO: a "pilemark", analogous to "petmark", perhaps a green plus sign,
which can be used by windowports with tiles.

win/tty/wintty.c

@@ -3040,7 +3040,7 @@ int glyph, bkglyph;
 
     /* must be after color check; term_end_color may turn off inverse too */
     if (((special & MG_PET) && iflags.hilite_pet)
-        || ((special & MG_DETECT) && iflags.use_inverse)) {
+        || ((special & (MG_DETECT|MG_OBJPILE)) && iflags.use_inverse)) {
         term_start_attr(ATR_INVERSE);
         reverse_on = TRUE;
     }