ether_demux does not handle frames with embedded vlan tags

[Ryan Stone]

ether_demux currently assumes that all vlan-tagged packets that it
sees have had the vlan stripped out and the M_VLAN tag is set, so it
never checks the ether type for a vlan.  However ng_ether_rcv_upper
currently does not guarantee that this is the case(and there may be
other code paths where this is also true).  Does anybody have any
strong feelings as to where the fix should go?  Making ether_demux
handle it is guaranteed to catch all cases but it does add a bit more
overhead to check for a vlan tag at each stage.