no such var: user-emacs-directory

[Thien-Thi Nguyen]

`scheme-start-file' in cmuscheme.el now references `user-emacs-directory'
but that variable is never defined anywhere.  the NEWS blurb mentions it,
although the docstring says "~/.emacs.d".

if the fix is to define `user-emacs-directory', probably many places that
hardcode "~/.emacs.d" could benefit from using the new variable.  but
that's more work than simply hardcoding it in cmuscheme.el as well (and
updating NEWS), so i propose to do the latter unless there are objections.

thi