Fix colorspace calculations on Haiku

Fix colorspace calculations on Haiku

* src/haiku_support.cc (be_get_display_planes)
(be_get_display_color_cells): Handle grayscale basic colorspaces
correctly.
(be_is_display_grayscale): New function.

* src/haiku_support.h: Update prototypes.

* src/haikufns.c (haiku_set_no_accept_focus, haiku_iconify_frame)
(Fxw_display_color_p, Fxw_color_values, Fx_display_grayscale_p):
Actually handle grayscale colorspaces.
(Fx_display_pixel_width, Fx_display_pixel_height)
(Fx_display_mm_height, Fx_display_mm_width): Clean up coding
style.
(Fx_display_visual_class): Handle grayscale colorspaces.
(syms_of_haikufns): New defsyms.