Compute frame workareas on Haiku

Compute frame workareas on Haiku

* lisp/frame.el (display-monitor-attributes-list): Implement
specially on Haiku as well.
* src/haiku_support.cc (get_zoom_rect): New function.  Extract
CalculateZoomRect here.
(class EmacsWindow, SetFullscreen): Use that instead of
CalculateZoomRect.
(be_get_explicit_workarea): New function.
* src/haiku_support.h: Update prototypes.
* src/haikufns.c (Fhaiku_display_monitor_attributes_list): New
function.
(syms_of_haikufns): Register new subr.