958e54f6a6
Additive backup_target field, true for the drive backing the PRIMARY tier. The controller cannot work this out itself: settings.StoragePath.BackupTarget is customer INTENT, and on the two boxes migrated by hand in E-1 that intent was never recorded -- intent is empty while the drive really IS the target. Without this flag the absent-target alarm could not name the drive on exactly the boxes that currently have one. omitempty + false on an older agent, so an old controller degrades to the generic disconnect alarm rather than a wrong one. Test asserts the target IS flagged AND the non-target is NOT, as a pair -- a blanket true would satisfy a naive one-sided check.