Add MY_COLLATION_HANDLER functions min_str() and max_str()

  /*

    Make minimum and maximum strings for the collation.

    Put not more than "nchars" characters.

  */

  size_t (*min_str)(CHARSET_INFO *cs, uchar *dst, size_t dstlen, size_t nchars);

  size_t (*max_str)(CHARSET_INFO *cs, uchar *dst, size_t dstlen, size_t nchars);