Switch direct rt fields access in rtsock.c to newly-create field acessors.

Switch direct rt fields access in rtsock.c to newly-create field acessors.

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.

Differential Revision:	https://reviews.freebsd.org/D27451