Don't append in setting up font-lock-keywords

otherwise the font locking is removed by the matcher for time, for
example:

    C3 正月 2011 13:30 座谈会

The date won't be properly highlighted.
Add font locking support for Chinese dates

through diary-mode-hook as suggested in
(info "(elisp)Customizing Keywords").
For #1: Compatible with http://debbugs.gnu.org/17393