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.

Passed senv->script_context rather than senv itself. Somehow the compiler fails to warn about this.

Used sieve_script_create instead of sieve_script_create_open.
Changed plugin debug output a bit as well.

Used errno instead of error variable.

Very useful to show that debugging is enabled and what version of pigeonhole is actually running.

Made normal compile error an info log item in the administrator log.

Forgot to increment count for the clone.

Setting skip_newline is useless when the connection is transfered to another process after successful login.

If a duplicate is detected the implicit keep is canceled, as though the redirect was successful. This prevents getting local deliveries.
The original SMTP recipient is used when it is available to augment the entry in the LDA duplicate database. This way, duplicates are only detected when (initially) addressed to the same recipient.
The main goal of the duplicate detection is mail loop prevention, but this also has the effect that mere duplicate deliveries are handled specially whithout a good reason. We should fix this in a future version.

By Timo Sirainen.

Added LIBDOVECOT_STORAGE to library dependencies in src/lib-sieve/Makefile.am.

lib-sieve: vacation extension: Fixed determination of From: address for when sieve_vacation_dont_check_recipient is active.

lib-sieve: vnd.dovecot.duplicate extension: Only track duplicate when Sieve script executes successfully.

Forgot to initialize variable, causing a segfault at runtime.

It is now possible to track duplicates based on arbitrary headers or even arbitrary string values using the new :header and :value arguments respectively.
The experation time can be configured using the new :seconds argument.
This change is backwards compatible as long as the name argument wasn't used. This is now a :handle <handle> argument.

Forgot to skip line upon error.

Added means to log user errors/warnings as info in master log.
Previous change was inadequate because an error was still logged.

Prevents administrator frustration about useless log messages caused by users with too much mail.

lib-sieve: include extension: didn't get semantics of mixing optional and non-optional includes for the same script quite right.

Change backported from Pigeonhole v0.4 tree, which means that this change will duplicate there upon merge.

Added support for the :optional tag of the include command.
Changed Sieve script API in the process; opening the script is now available as a separate step.