Patch #71684 by fgm: corrected the header information of RSS feeds: text/xml -> application/rss+xml, backport from HEAD

backport from HEAD

   * 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.

- Patch #42068 by mfb et al: fixed problem with aggregator destroying certain links.  Moved filtering to on output.  (Today's critical bugfix #2.)

  Changed code to be compatible with all versions.

  TODO:
  + The contact.module was broken; a new patch for contact.module is needed.
  + Documentation is needed.
  + The most important modules need to be updated ASAP.

* adds a "feed settings" section to admin/settings where 2 new settings are introduced:
* number of items per feed
* default length of feed descriptions (title only, teaser, full)
* patches all of core to obey the above - including the new aggregator (out) feeds
* adds support for adding namespaces in _nodeapi('rss item') - which means things like iTunes RSS and yahoo's media rss can be implemented by the appropriate modules (i.e. audio.module)
* includes some additional info in the default node feed - specifically the element (links directly to comments) - and dc:creator - to show node author information.