RFC 4861 4.1 allows router solicitations from the unspecified address

RFC 4861 4.1 allows router solicitations from the unspecified address
(::). A short survey suggests that some systems respond with a
multicast router advertisement and other don't.

Andrew Forgue pointed out a usecase for this, booting a UEFI system
over http in an IPv6 network only. ("IPv6 support for UEFI IPv6
booting" on bugs@ a long time ago)

There is also no downside in allowing this, any host on the layer 2
network can solicit a router advertisement.

OK phessler