Support moons as candidate links in bonding/multipath

Currently, according to the documentation, the bonding/multipath feature only supports configurations on physical (system) links. However, suppose the user has one or several dedicated moons which have better routes to other nodes, the user may want to specify those moons as candidate routes, while the physical interface remains the same one. This is also useful when the direct connection is unstable (e.g., due to ISP QoS), and the user wants to use his/her own moon(s) as failover.