bug#7296: display-pixel-height not enough

[Lennart Borgman]

If you want to know how much height there is available to display a
frame then display-pixel-height does not give you the information you
need. The taskbar (w32 name, I have no idea what it is called on other
platform) and other "bars" may have reserved some of the vertical
space.

I suggest adding something like display-available-pixel-height that
gives the actual height available for a frame that can be totally
visible.

(There is a similar problem for the width.)