zmachine: game file name is hardcoded in frotz.conf

The checked-in zmachine/frotz.conf has a hard-coded reference to LostPig.z8, which obviously will only work for Lost Pig.  (It is important to keep the game's proper filename on the ISO itself, and not use a generic filename).  This filename should instead be replaced with the filename of the game image of whatever game ISO is being built.
