MFC ng_ether support for if_bridge and cleanup of bridge hooks.

bridgestp.c; r1.8
if_bridge.c; r1.25, 1.27
if_bridgevar.h; r1.5
if_ethersubr.c; r1.201, 1.202, 1.206
ng_ether.c; r1.52 - 1.54

Approved by: re (scottl)