DefaultTailscaledSocket returns the path to the tailscaled Unix socket or the empty string if there's no reasonable default.

DefaultTailscaledStateFile returns the default path to the tailscaled state file, or the empty string if there's no reasonable default value.

LegacyStateFilePath is not applicable to UNIX; it is just stubbed out.

MkStateDir ensures that dirPath, the daemon's configurtaion directory containing machine keys etc, both exists and has the correct permissions.

AppSharedDir is a string set by the iOS or Android app on start containing a directory we can read/write in.