Persistently store NIC's hardware MAC address, and add a way to retrive it

jhb pointed out that (struct ifnet) is part of the network driver KBI, and
thus the offsets of internal fields must not change. Therefore, move the new
"if_hw_addr" field to the end, and consume one of the "if_pspare"s; that's
what they're there for. The new field replaces the *last* element of that
array; that way, offsetof(if_pspare) and offsetof(if_ispare) are unchanged
compared to before r318397.

PR: 194386
Reviewed by: jhb
Pointyhat to: rpokala
Sponsored by: Panasas