Improve portion of the dialog(1) API in dialog.subr responsible for
calculating widget sizes. Instead of forking a sub-shell to calculate the
optimum size for a widget, use a byRef style call-out to set variables in
the parent namespace. For example, instead of:

	size=$( f_dialog_buttonbox_size title btitle msg )
	$DIALOG --title title --backtitle btitle --msgbox msg $size

The new API replaces the above with the following:

	f_dialog_buttonbox_size height width title btitle msg
	$DIALOG --title title --backtitle btitle --msgbox msg $height $width

This reduces the number of forks, improves performance, and makes the code
more readable by revealing the argument-order for widget sizing. It also
makes performing minor adjustments to the calculated values easier as
you no longer have to split-out the response (which required knowledge of
ordering so was counter-intuitive).