MFC r342168,357786: Allow ng_nat to be attached to a ethernet interface

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).

Submitted by: sobomax
Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D18535
Relnotes: yes