Rename variable name from 'index' to 'idx' to avoid shadowing index(3).

Noticed by:	dim
MFC after:	2 weeks
Move struct ether_vlan_header to ethernet.h, out of if_vlan_var.h,
since this structure is protocol definition, not part of implementation.
Merge remote-tracking branch 'freebsd/master' into hardened/current/master