[PATCH master] IPv6 ECMP support fixes for linux

[Mikhail Sennikovskii]

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