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.

Using a location option for the default name makes no sense if its only used for the main personal script.