(nnmail-insert-xref): Encode pathname for Emacs 20.
(nnmail-write-region, nnmh-retrieve-headers, nnmh-request-article,
nnmh-request-group, nnmh-possibly-change-directory): Protect from
conversion by `pathname-coding-system' for XEmacs/mule.
(nnmail-pathname-coding-system): New variable.
(nnmail-group-pathname): Encode pathname for Emacs 20.
nnmail-file-coding-system): New variable.
(nnmail-find-file): Bind `coding-system-for-read' with
`nnmail-file-coding-system' for Emacs/mule and XEmacs/mule.
(nnmail-write-region): Bind `coding-system-for-write' with
`nnmail-file-coding-system' for Emacs/mule and XEmacs/mule.
