Annotate branches for fixes for CVE-2019-16159

Issues likely only affect versions >= 1.6.4 for the 1.6.x series and >=
2.0.0 for the 2.0.x series but needs to be checked. Information is
available in

http://trubka.network.cz/pipermail/bird-users/2019-September/013720.html

and

http://trubka.network.cz/pipermail/bird-users/2019-September/013722.html