Fix active backup link selection

[william-swarmbotics]

Tested with the "always" link selection policy and a designated primary link.

Old behavior: When the primary link goes down and comes back up, Zerotier does not switch back to it. New behavior: Zerotier does switch back to the primary link.

[CLAassistant]

All committers have signed the CLA.

[Tokisaki-Galaxy]

useful commit!