paths: add synology socket path

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

paths/paths.go

@@ -26,6 +26,13 @@ func DefaultTailscaledSocket() string {
 	if runtime.GOOS == "darwin" {
 		return "/var/run/tailscaled.socket"
 	}
+	if runtime.GOOS == "linux" {
+		// TODO(crawshaw): does this path change with DSM7?
+		const synologySock = "/volume1/@appstore/Tailscale/var/tailscaled.sock" // SYNOPKG_PKGDEST in scripts/installer
+		if fi, err := os.Stat(filepath.Dir(synologySock)); err == nil && fi.IsDir() {
+			return synologySock
+		}
+	}
 	if fi, err := os.Stat("/var/run"); err == nil && fi.IsDir() {
 		return "/var/run/tailscale/tailscaled.sock"
 	}