[PATCH master] IPv6 ECMP support fixes for linux

Hello Fellows, as a follow-up for the recent discussion about BIRD support of IPv6 ECMP on Linux ( http://trubka.network.cz/pipermail/bird-users/2016-January/010163.html ), I've created a patch, which addresses this problem. The current patch is against the master branch, i.e. for BIRD 1.x. I will also post a similar patch against the int-new branch, for BIRD 2. Note that the patch still does not support the learn mode for IPv6 ECMP. The support for it can be added later. Thanks, Mikhail