Use the provided `tailscaled.service` file and override what we need to

[legowerewolf]

The distributed tarball provides a service unit file that's almost correct for our needs.
Specifically, the provided file:

• Doesn't list the ExtensionDirectories path we need
• References an EnvironmentFile (also distributed)
• References variables from the EnvironmentFile in ExecStart

Therefore:

• Copy that service file into the right place
• Copy the provided EnvironmentFile into the right place (do not replace if it exists)
• Copy/create an override.conf file in the right place to add the ExtensionDirectories path (do not replace if it exists)