emacs: Extend face to window edge again

emacs: Extend face to window edge again

Since Emacs 27 each face has to be explicitly configured to "extend
to the edge of the window".  Without doing that the face used for
the newline character only has an effect that spans "one character"
(i.e. it looks like there is a single trailing space character).

We don't want that so extend the face in Emacs 27, so that it looks
the same as it did in older Emacs releases.  We have to do this
conditionally, otherwise older Emacsen would choke on it.