netlink: add snl(3) default parsers for routes and links.

This change adds the "default" parsers of _all_ route/link attributes
exported by the kernel.
It removes the need to declare similar parsers in the userland applications,
simplifying their logic.

Differential Revision: https://reviews.freebsd.org/D38979
MFC after: 2 weeks

(cherry picked from commit 595d23f777dc24cb285b072c596ca96bbc2db3cd)