multi-tty suspend-hook

[Dan Nicolaescu]

emacs -q -nw -f server-start

use emacsclient to create an X11 frame. This runs
x-initialize-window-system which does
  (add-hook 'suspend-hook 'x-win-suspend-error)

Quit the X11 frame and press C-z in the original tty frame.  It thows an
error: "Suspending an Emacs running under X makes no sense"
Which makes no sense (:-), there's no X frame running at this time.

Adding a check in x-win-suspend-error to check if the frame is x fixes
this. But is this TRTD?