Committed Timo's patch that adds a plugin that provides Sieve dsync support.

http://dovecot.org/patches/2.2/doveadm-sieve-plugin.diff
Made some small modifications.
This code should not interfere with normal ManageSieve operation, so I commit this right now without much testing.
The actual doveadm sieve plugin has a few open issues that need to be resolved still.