The actual version numbers live only on the release-branches from now on.

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.

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

Doveadm-sieve plugin may be broken at this point; not tested so far.

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

In the future, much of these can be moved to Dovecot.

This is still an unfinished feature.