NAME
	file_name - find the filename of an object

SYNOPSIS
	string file_name(object ob)

DESCRIPTION
	This function gives the filename of an object concatenated with
	a possible instance number. This typically is a string on the
	form "/std/weapon#423". Instance numbers are unique for all
	objects in the game.

	If a filename has no instance number it is the 'master object' and
	not a 'cloned copy'.