rtsock code was build around the assumption that each rtentry record in the system radix tree is a ready-to-use sockaddr.
This assumptions turned out to be not quite true:
* masks have their length tweaked, so we have `rtsock_fix_netmask` hack
* IPv6 addresses have their scope embedded, so we have another explicit deembedding hack.
Change the code to decouple rtentry internals from rtsock code using newly-created rtentry accessors.
This will allow to eventually eliminate both of the hacks and change rtentry dst/mask format.