MFC r272401, r272402: Rework the logic of sequential SCSI LUN scanner.

Previous logic was not differentiating disconnected LUNs and absent targets.
That made it to stop scan if LUN 0 was not found for any reason. That made
problematic, for example, using iSCSI targets declaring SPC-2 compliance and
having no LUN 0 configured.

The new logic continues sequential LUN scan if:

• we have more configured LUNs that need recheck;
• this LUN is connected and its SCSI version allows more LUNs;
• this LUN is disconnected, its SCSI version allows more LUNs and we guess they may be connected (we haven't scanned first 8 LUNs yet or kern.cam.cam_srch_hi sysctl is set to scan more).