netlink: add rtsock-compatible header to use with netlink snl(3).

Some routing socket defines (RTM_ and RTA_ ones) clash with the ones
used by the the Netlink.
As some rtsock definitions like interface flags or route flags are used in
both netlink and rtsock, provide a convenient way to include those without
running into the define collision.

Differential Revision: https://reviews.freebsd.org/D38982
MFC after: 2 weeks