This adds the vnd.dovecot.default-mailbox and vnd.dovecot.username environment items.
It creates the 'env' variables namespace through which all environment items can be accessed directly.
This extension is yet to be documented properly.
More environment items are under consideration.

Also added testsuite items to test syntax.

Implmented reporting full Pigeonhole version with hg tag so that builds between releases can be distinguished.
Copied from Dovecot for the most part.

Backwards compatibility is provided for vnd.dovecot.duplicate.
Still need to fix the constraint that it must not track duplicates from failed Sieve executions.

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

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.

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

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

Spamtest and virustest extensions: discarded whitespace at beginning of regexp and added syntax testsuite tests.

Fixed small bug in the i;ascii-numeric comparator and added extensive relational tests to the testsuite.