int on_drop(object player)

Called whenever your object is dropped by the player who was holding it.
player will often be equal to this_player(), but you shouldn't assume that
(for example, a player might have a "force to drop" spell). The return
value matters, and possible results are defined in const.h:

0 - Do nothing special. The drop fails.
MOVE_OK_FLAG - The drop succeeds. The object will drop.
MOVE_SILENCE_FLAG - Do not mention that the object was dropped. This
assumes that you printed your own specialized message (i.e. "The object
dissolves into dust as you drop it.") and you don't want the default
one to be printed.

You can bitwise-OR flags (i.e. 'MOVE_OK_FLAG | MOVE_SILENCE_FLAG') to
combine effects. item.c implements a default on_drop handler. This function
is not called when a 'give' occurs (see on_give).

