Is now intended to the used for user deinitialization.
This fixes mail_user_session_finished event not being sent
with managesieve.

To prevent a maybe-uninitialized warning the autodetecting now sets the
storage_path parameter in any case.

These were originally written while the plugin was in draft. These can
be now removed as the RFC has been finalized and the commands were
dropped.

lib-sieve: sieve-storage, sieve-file-storage - Drop deprecated handling of 'sieve_dir' configuration

It only causes confusing output.

The dump-capability just wants to get the capabilities. Any problems with
settings will be handled later on.

Also change the callers to not bother storing it separately. Instead, they
can just use mail_user_r->service_user.

It must not be committed

Needed for adding a per-mailbox event in later commit.