Log Message:
Update mail/mairix to 0.21

Release notes:

* Fix make clean target in dfasyn/ (Benj. Mako Hill)
* Limit number of messages that are examined when an end boundary is
  missing in an mbox (Chung-chieh Shan)
* Avoid examining . and .. when traversing MH folder hierarchy (Steven Lumos)
* Fix various bugs in the name/value parser
* Add some RFC2231 support to the name/value parser (continuations)
* Fix indexing when existing database only contains 1 message