* riece-unread.el: Use riece-facep to check existence of

* riece-unread.el: Use riece-facep to check existence of
'riece-modeline-unread-face.
* riece-history.el: Use riece-facep to check existence of
'riece-modeline-history-face.
* riece-highlight.el: Use riece-facep to check existence of
'riece-modeline-current-face.

* riece-xemacs.el (riece-facep): Define as an alias of 'find-face.
* riece-emacs.el (riece-facep): Define as an alias of 'facep.