extprograms plugin: Clarified the language specification regarding handling of filter and execute errors.

It allows sending feedback report (RFC 5965) messages.

This was quite fundamentally broken, not only due to previous change.

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.

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.

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.

Also fixed a small mistake in the spefication for the vnd.dovecot.duplicate extension.

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