cmd/tailscale, util/quarantine: set quarantine flags on files from Taildrop

This sets the "com.apple.quarantine" flag on macOS, and the
"Zone.Identifier" alternate data stream on Windows.

Change-Id: If14f805467b0e2963067937d7f34e08ba1d1fa85
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

util/quarantine/quarantine_default.go

@@ -0,0 +1,15 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !darwin && !windows
+
+package quarantine
+
+import (
+	"os"
+)
+
+func setQuarantineAttr(f *os.File) error {
+	return nil
+}