Support linux in from_getifaddrs

On BSD-like operating systems the raw interface has a protocol family of
PF_LINK.  On Linux a protocol family of PF_PACKET is used instead.
macaddr 1.7.0 did not have linux support and raised an exception when
trying to discover the MAC address via Socket.getifaddrs.

Now we handle both PF_LINK and PF_PACKET.

Fixes #24