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.

include/hack.h

@@ -60,6 +60,7 @@
 #define MG_PET 0x08
 #define MG_RIDDEN 0x10
 #define MG_STATUE 0x20
+#define MG_OBJPILE 0x40  /* more than one stack of objects */
 
 /* sellobj_state() states */
 #define SELL_NORMAL (0)