[os-tailscale] Support tailscale set arguments

[AlexRusbridge]

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Related issue

• Closes Tailscale - add peer relay function #5122

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Describe the problem

Tailscale have a preference for adding new flags to the tailscale set command, instead of tailscale up.

// When adding new flags, prefer to put them under "tailscale set" instead
// of here. Setting preferences via "tailscale up" is deprecated.

up.go

This means that any new functionality added to tailscale is likely to be configured through set. In the plugins current state, we can't configure these options.

We can already see the impact with the --relay-server-port flag.

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Describe the proposed solution

This PR adds a new rc.d script tailscaled_set that will apply tailscale set flags after the tailscaled service has started.

It supports defining arguments via the tailscaled_set_args variable.

For now, we can configure --relay-server-port and --webclient via the UI. But this PR opens up the possibility of more set arguments.

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Note: There is another solution, where the FreeBSD port of tailscaled could support tailscaled_set_args alongside it's tailscaled_set_args.

[sheridans]

This looks like a promising idea, let me have a look over it and ensure nothing breaks.