Allow ng_nat to be attached to a ethernet interface directly via ng_ether(4)
or the likes. Add new control message types: setdlt and getdlt to switch
from default DLT_RAW (no encapsulation) to DLT_EN10MB (ethernet).

Approved by: glebius
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D18535