ether: add older ethertype definitions for QinQ

Older network equipment used the ethertypes 0x9100, 0x9200, and 0x9300 for
outer VLANs, before standardisation introduced 0x88a8.

Submitted by: Lutz Donnerhacke <lutz_donnerhacke.de>
Differential Revision: https://reviews.freebsd.org/D21846