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.

sieve-filter: Fixed handling of failure-related implicit keep when there is an explicit default destination folder.
The message was left in the source mailbox, even though it was stored in the destination folder.

Previous change had several problems.

It now uses dsasl-client, so it should now be possible to use SASL mechanisms other than PLAIN.
Cleaned up the code a bit.

managesieve: Managesieve used "managesieve" rather than "sieve" as login service name, which means that all managesieve-specific settings where ignored.

Nobody does that, but we support it.

When the default was first overriden, then deactivated and finally overriden again, it would erroneously implicitly re-activate.

lib-sieve: storage: Added parameter to sieve_script_delete() to allow ignoring the active status of the script.
Normally, the active script cannot be deleted.
This change is necessary to handle doveadm sieve delete -a correctly when the provided script is the default.

It only worked when sieve_default_name was set.

Caused by earlier sieve_default_name-related changes.

This caused a segfault in the test suite.