[Bug: 21.5-b27] modeline problem: frame-tty-p not defined

[Stephen J. Turnbull]

Ron Isaacson writes:

 > In modeline.el, modeline-update-tty-frame-specifier tries to use
 > frame-tty-p. If that function is not defined, modeline-tty-frame-id
 > will always be blank, and it doesn't seem to be defined anywhere.

Why not just use (eq (frame-type f) 'tty)?  AFAICT this isn't used
anywhere else, nor do corresponding type-specific predicates exist for
other kinds of frames.