Allow listening on tailnet and local network

[grantstephens]

I would like to run a single golink service that listens on the tailnet, but also on the local network so that devices not part of the tailnet can use the service.
Having a brief look at the code this may be as simple as allowing the dev listen and service to run in a goroutine and then continuing on with the other options, but a new flag would probably be required in order to achieve that.