Value is `tty' for a tty frame (a character-only terminal),
`x' for a frame that is an X window,
`ns' for a frame that is a NeXTstep window (not yet implemented),
Value is `tty' for a tty frame (a character-only terminal),
`x' for a frame that is an X window,
`ns' for a frame that is a NeXTstep window (not yet implemented),