MFC r349321: Improve AHCI Enclosure Management and SES interoperation.

Since SES specs do not define mechanism to map enclosure slots to SATA
disks, AHCI EM code I written many years ago appeared quite useless,
that always bugged me. I was thinking whether it was a good idea, but
if LSI HBAs do that, why I shouldn't?

This change introduces simple non-standard mechanism for the mapping
into both AHCI EM and SES code, that makes AHCI EM on capable controllers
(most of Intel's) a first-class SES citizen, allowing it to report disk
physical path to GEOM, show devices inserted into each enclosure slot in
sesutil map and getencstat, control locate and fault LEDs for specific
devices with sesutil locate adaX on and sesutil fault adaX on, etc.

I've successfully tested this on Supermicro X10DRH-i motherboard connected
with sideband cable of its S-SATA Mini-SAS connector to SAS815TQ backplane.
It can indicate with LEDs Locate, Fault and Rebuild/Remap SES statuses for
each disk identical to real SES of Supermicro SAS2 backplanes.

Relnotes: yes