Various changes to feed.php’s parameter handling

  * Do not emit E_NOTICEs
  * Introduce explicit feed mode default value »recent« instead of handling
    every unknown feed mode as recent changes mode. This means:
    - FEED_MODE_UNKNOWN is only executed if the given feed mode is indeed
      unknown. This is specifically not the case if no feed mode request param
      has been specified or the requested feed mode is »recent«.
    - FEED_MODE_UNKNOWN dies with an error as default behaviour
  * opt[items] is forced to be a non-negative integer
  * opt[show_minor] is forced to be a boolean and thus has a default value of
    false instead of null