netlink: add netlink interfaces to if_clone

This change adds netlink create/modify/dump interfaces to the if_clone.c.
The previous attempt with storing the logic inside netlink/route/iface_drivers.c
did not quite work, as, for example, dumping interface-specific state
(like vlan id or vlan parent) required some peeking into the private interfaces.

The new interfaces are added in a compatible way - callers don't have to do anything
unless they are extended with Netlink.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D39032
MFC after: 1 month