conn: introduce new package that splits out the Bind and Endpoint types

The sticky socket code stays in the device package for now,
as it reaches deeply into the peer list.

This is the first step in an effort to split some code out of
the very busy device package.

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

conn/mark_default.go

@@ -0,0 +1,12 @@
+// +build !linux,!openbsd,!freebsd
+
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2017-2019 WireGuard LLC. All Rights Reserved.
+ */
+
+package conn
+
+func (bind *nativeBind) SetMark(mark uint32) error {
+	return nil
+}