[cmaster-next] support for capnproto API on Quagga

Hi all, I am working on some support of capnproto interface on Quagga (following David's contribution). In order to avoid duplication of effort, I would like first to get your review from cmaster-next community, The code can be seen here: https://lists.quagga.net/pipermail/quagga-dev/2016-December/016521.html then, based on your reviews, I'll update the patch series and I'll work to rebase it on cmaster-next too. Thanks, Regards, Philippe