cmd/trayscale: exit nodes don't always work

[DeedleFake]

The Tailscale CLI tool requires all non-default options to be re-specified whenever running tailscale up to change something. This means that if the user has previously run tailscale up --advertise-exit-node, running tailscale up --exit-node=1.2.3.4 will fail unless --advertise-exit-node is also specified again. Alternatively, the user can specify --reset to set all options that aren't specified back to their defaults.

The best option is to find some way to find out what options are set and setting them alongside the others. If necessary, it might be possible to parse the required options out of the error message printed by the call to tailscale up.