Adds a new form attribute that lets form writers hook into a conveniant place when the form building requires knowledge of the validation results.

edited for some formatting issues.

- Patch #51850 by chx, webchick et al: fixed various problems with db_rewrite_sql, made db_rewrite_sql slightly more robust.

   * fapi conversion.
   * Adds status messages, comments and language fixes.
   * Adds watchdog messages for category and feed additions and deletions.
   * Edit forms: checks for duplicates - title for category, title + url for feeds.

    * converts forum_admin to fapi.
    * handles invalid terms in hook_menu
    * adds comments, fixes typos etc.
    * reorganises functions a little bit - moves hook_menu to the top.. groups hooks together and forms together