safesocket: add js/wasm implementation with in-memory net.Conn

Updates #3157

Change-Id: Ia35b1e259011fb86f8c4e01f62146f9fd4c9b7c6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

safesocket/safesocket_js.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2021 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 safesocket
+
+import (
+	"net"
+
+	"github.com/akutz/memconn"
+)
+
+const memName = "Tailscale-IPN"
+
+func listen(path string, port uint16) (_ net.Listener, gotPort uint16, _ error) {
+	ln, err := memconn.Listen("memu", memName)
+	return ln, 1, err
+}
+
+func connect(path string, port uint16) (net.Conn, error) {
+	return memconn.Dial("memu", memName)
+}