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.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.
+
+// Package quarantine sets platform specific "quarantine" attributes on files
+// that are received from other hosts.
+package quarantine
+
+import "os"
+
+// SetOnFile sets the platform-specific quarantine attribute (if any) on the
+// provided file.
+func SetOnFile(f *os.File) error {
+	return setQuarantineAttr(f)
+}