Revision 318160

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

The MAC address reported by `ifconfig ${nic} ether' does not always match
the address in the hardware, as reported by the driver during attach. In
particular, NICs which are components of a lagg(4) interface all report the
same MAC.

When attaching, the NIC driver passes the MAC address it read from the
hardware as an argument to ether_ifattach(). Keep a second copy of it, and
create ioctl(SIOCGHWADDR) to return it. Teach `ifconfig' to report it along
with the active MAC address.

PR:		194386
Reviewed by:	glebius
MFC after:	1 week
Sponsored by:	Panasas
Differential Revision:	https://reviews.freebsd.org/D10609