[Gnus] XEmacs 21.5 compilation fix

[Gnus] XEmacs 21.5 compilation fix

* gnus-score.el (gnus-score-decode-text-parts): Use #' for
  mm-text-parts used in labels macro to make it work with XEmacs 21.5.
* gnus-util.el (gnus-string-prefix-p): New function, an alias to
  string-prefix-p in Emacs >=23.2.
* nnmaildir.el (nnmaildir--ensure-suffix, nnmaildir--add-flag)
  (nnmaildir--remove-flag, nnmaildir--scan): Use gnus-string-match-p
  instead of string-match-p.
  (nnmaildir--scan): Use gnus-string-prefix-p instead of string-prefix-p.