Recent changes in handling of unknown tags caused `make test-plugins' to fail.

Also added testsuite items to test syntax.

lib-sieve: Flush duplicate database during start phase of result execution rather than commit phase.
This will release the duplicate database lock earlier, even before a mail storage transation is started.

This allows extension of test commands that evaluate the message somehow.
Currently this is implemented only for the message header.
Extensions can thereby influence what headers are evaluated and how.

lib-sieve: Implemented the index stringlist class which selects an item of the parent at a particular index.

This is needed so that argument validation functions can reliably verify other arguments at arbitrary positions.

The sieve_result_execute() function returned the wrong status result.

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

This was caused by a circular reference.