Introduce before queue dstatus

Introduce before queue dstatus

Currently the status of a before queue filtered mail is the status of the
downstream delivery in case the downstream rejects or defers a mail.

This information is too little to distinguish between a mail which went
through the rule-system, and a locally generated one
(e.g. bounce or spamreport).

By introducing 3 new distinct dstatus values we can use the information to
correctly display the status of a mail in the GUI.

Reviewed-By: Mira Limbeck <m.limbeck@proxmox.com>
Tested-By: Mira Limbeck <m.limbeck@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>