Add Dovecot auth

This adds code to authenticate against a Dovecot server *without* resorting to IMAP. It uses [Dovecot's Authentication Protocol][1] to communicate directly with Dovecot through a local/unix socket. This is functionally similar to the Courier auth module.

The primary advantage of using this instead of the IMAP module is that a CalDAV sync doesn't cause syslog to be spammed with lots of login/logout events in a short amount of time (I've seen bursts of 10/sec sustained for about a minute during initial download of a calendar).

[1]: http://wiki2.dovecot.org/Design/AuthProtocol