TTY frames parameters

[Eli Zaretskii]

There's a strange subtle difference between the first tty frame
created on a default terminal and all the other frames: the first one
has its `tty' association with nil cdr, while the rest have
"/dev/tty".  Is that a bug?

To reproduce:

  emacs -Q -nw
  M-: (frame-parameter (selected-frame) 'tty) RET => nil
  C-x 5 b foo
  M-: (frame-parameter (selected-frame) 'tty) RET => "/dev/tty"

You will also see that in the initial frame, (frame-parameters)
returns an alist that contains `(tty)', not `(tty . "/dev/tty")' as in
the other frames.