wrong fullscreen frame size on osx

[Yuri D'Elia]

When running carbon emacs (21.2.1) on osx with dual monitor setup, the 
fullscreen option gives incorrect results. Apparenty emacs sets the 
width (can't test the height) of the frame using the full size of both 
monitors instead of the current console only.

The positioning on the other hand seems correct (the frame remains on 
the current screen).

Makes (fullscreen . fullboth) usage completely useless.
EDiff maximize also screws off badly for the same reason.