kernel-netlink: Implement passthrough type routes and use them on Linux

kernel-netlink: Implement passthrough type routes and use them on Linux

Enables us to ignore any future kernel features for routes unless
we actually need to consider them for the source IP routes.

Also enables us to actually really skip IPsec processing for those networks
(because even the routes don't touch those packets). It's more what
users expect.

Co-authored-by: Tobias Brunner <tobias@strongswan.org>