Via the flags of the RTM_NEWROUTE netlink message, kernel and iproute2
support various variants to add a route.

 - ip route add
 - ip route change
 - ip route replace
 - ip route prepend
 - ip route append
 - ip route test

Previously, our nm_platform_ip4_route_add() function was basically
`ip route replace`. In the future, we should rather user `ip route
append` instead.

Anyway, expose the netlink message flags in the API. This allows to
use the various forms, and makes it also more apparent to the user that
they even exist.