Disambiguate "Packet Tag" from "Packet Type"

This non-substantive MR tries to remove the ambiguity around the term "Packet Tag".

"Packet Tag" now refers only to the first octet of the packet.

"Packet Type" is the category of packet, represented by a positive integer.

Since draft-09 is already released, this might not be worth merging. I certainly won't have my feelings hurt if it gets closed without being applied.