ipn/ipnserver paths: add paths.LegacyStateFilePath

Moving this information into a centralized place so that it is accessible to
code in subsequent commits.

Updates #3011

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

paths/paths_unix.go

@@ -81,3 +81,8 @@ func ensureStateDirPerms(dir string) error {
 	}
 	return os.Chmod(dir, perm)
 }
+
+// LegacyStateFilePath is not applicable to UNIX; it is just stubbed out.
+func LegacyStateFilePath() string {
+	return ""
+}