(nnheader-file-coding-system): New variable.
(nnheader-group-pathname): Encode pathname for Emacs 20.
(nnheader-find-file-noselect): Bind `coding-system-for-read' with
`nnheader-file-coding-system' for Emacs/mule and XEmacs/mule.
(nnheader-insert-file-contents): Bind `coding-system-for-read'
with `nnheader-file-coding-system' for Emacs/mule and XEmacs/mule.