ofproto-dpif: Separately track the initial VLAN TCI of arriving packets.
In an upcoming commit, VLAN splinters can cause the VLAN TCI of a packet
received on an interface to differ from the logical VLAN TCI. That is,
a packet that is received on a Linux VLAN network device has no VLAN (so
its initial VLAN TCI is 0) but we logically treat it as if it has the VLAN
associated with the VLAN device.
This is only desirable for use with VLAN splinters and should be reverted
when this feature is no longer needed. I'm breaking it out here only to
make the series easier to review.