I've a distant computer that sets the DISPLAY value on ssh, but it has
a so old X server that Emacs can't connect to it. No problem, it's not
due to Emacs.

However, I then use `emacs -nw'.

Well, I don't see the point of this message :

  $ emacs -nw
  Xlib: connection to "localhost:10.0" refused by server
  Xlib: Invalid MIT-MAGIC-COOKIE-1 key
  Display localhost:10.0 unavailable, simulating -nw
  $

Emacs, my dear, please don't simulate, do it.

I propose the following (maybe naive) tiny change :