Install routes after addresses are ready

If an IPv6 route is added with a source address that is still
tentative, the kernel will refuse to install it.

Previously, once we sent the messages to the kernel to add the
addresses, we would immediately proceed to add the routes. The
addresses would usually still be tentative at this point, so
adding static IPv6 routes was broken - see issue #5882.

Now, only begin to configure routes once the addresses are ready,
by restructuring the state machine, and tracking when addresses are
ready, not just added.

Fixes: #5882
Signed-off-by: Daniel Axtens <dja@axtens.net>