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.

[sfllaw]

This is kind of related to #95.

[sfllaw]

@grantstephens When devices that are not part of the tailnet connect to this service, I assume they should only have read-only access?

[grantstephens]

@grantstephens When devices that are not part of the tailnet connect to this service, I assume they should only have read-only access?

Yes, that would probably have to be the case, although, would be cool if that was optional.

[sfllaw]

@grantstephens When devices that are not part of the tailnet connect to this service, I assume they should only have read-only access?

Yes, that would probably have to be the case, although, would be cool if that was optional.

Sorry, do you mind clarifying what is the optional bit? If a client is not on a tailnet and is connecting to the golink service over the local network, what is the optional behaviour?

[grantstephens]

Sorry, I meant make the read only access optional, i.e. if I'm running it on my home network I'd be ok with all devices being able to have write access, however there may be cases where you'd be running it on the open internet and then read-only would be better.

[sfllaw]

I understand now. Thanks for the suggestion!