It's still not fully correct, but we don't have existing code that supports
parsing it.

Use mail_get_message_id() instead of just mail_get_first_header().

Previous commit was accidentally for an earlier version of the API change.

Found by latest clang scan-build.

Nothing changes in the code logic.

The old code seems to have worked correctly already, but now it's a bit
clearer how it was intended to work.

Fixes "warning: cast to smaller integer type" with new clang.

Default is now debug logging, which is used for the IMAP plugins. The LDA Sieve
plugin uses info logging.

When no insertion point was found, it would always insert the new rule at the
beginning, rather than at the end.

lib-sieve: plugins: vacation: cmd-vacation - Consolidate message address comparison in separate function.