This series replaces the previous patches in this thread, reposting them as
- patch 1/2: "ipv6: addrconf: fix mcast route for GRE devices"
which contains the semantic changes for the ff00::/8 route on GRE, and
- patch 2/2:  ipv6: addrconf: clean up device type handling"
which just moves things to make them prettier, and documents the "speshul"
fe80::/64 behaviour (which Nicolas Dichtel added in 08b4465 to make a specific
user case work.)

Tested by poking various devices with a bunch of link ups and downs.

Hannes, can you Ack?

-David