Phonet: PF_PHONET protocol family support

This is the basis for the Phonet protocol families, and introduces
the ETH_P_PHONET packet type and the PF_PHONET socket family.

Signed-off-by: Remi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>