Simplify fullscreen management on Haiku

Simplify fullscreen management on Haiku

* src/haiku_support.cc (class EmacsWindow, Zoom, UnZoom): Track
zoom state manually instead of guessing what the system
currently thinks it is.
(MakeFullscreen): Always unzoom first.
* src/haiku_support.h (struct haiku_zoom_event): Remove all
fields and add a single field `zoomed'.
* src/haikufns.c (haiku_create_frame, haiku_create_tip_frame):
Remove use of pending_zoom fields.
* src/haikuterm.c (haiku_read_socket): Simplify handling of zoom
events.
(haiku_fullscreen): Simplify handling of different zoom states.
* src/haikuterm.h (struct haiku_output): Remove all pending_zoom
fields since they are no longer required.