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

An earlier version of r318160 allocated if_hw_addr unconditionally; when it
became conditional, I forgot to check for NULL in ether_ifattach().

Reviewed by: kp
MFC after: 1 week
MFC with: r318160
Sponsored by: Panasas
Differential Revision: https://reviews.freebsd.org/D10678
Pointy-hat to: rpokala