(nnheader-decode-subject): New alias.

(nnheader-decode-subject): New alias.
(nnheader-decode-from): New alias.
(make-full-mail-header): Use `nnheader-decode-subject' and
`nnheader-decode-from' instead of
`eword-decode-and-unfold-unstructured-field' and
`eword-decode-and-unfold-structured-field'.