It is still a plugin, but no longer a separate package.

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.

Apparently automakes in future won't support configure.in anymore.

Incorporated sieve_duplicate plugin into main Pigeonhole tree as a normal extension (vnd.dovecot.duplicate).
Also restructured doc dir a bit in the process.

Updated version number to v0.4.0 and added a warning to configure.
Applied generic-login.patch.

Made vnd.dovecot.debug extension available to the LDA plugin instead of only the command line tools.

Explicitly set AC_CONFIG_AUX_DIR to prevent issues when compiling Pigeonhole in a sub-directory of the Dovecot tree.

Let configure automatically infer path to installed Dovecot and don't rely on installed libtool .la files anymore (patch by Timo Sirainen).