DISPLAY -- Prototype routines for loading and controlling the image display.
The lower level code is device dependent.

	display		loads the display
	dcontrol	adjusts the display (frame select, window, etc.)

The basic strategy is that the image display device is interfaced to IRAF 
file i/o as a binary file.  IMIO is then used to access the image or graphics
planes of the device as a disk resident imagefile would be referenced.
Each image plane of each image device is a separate "imagefile", and has a
distinct image header file in the directory "dev$".

This package uses the ZFIOGD (binary graphics device) device driver, the
source for which is in host$gdev.  It is this driver which implements physical
i/o to the device (actually, to the host system device driver for the device).
