Adopt GOPATH

GOPATH is annoying, but the Go community pushing me to adopt it is even
more annoying.

donotuseon_linux.go

@@ -0,0 +1,15 @@
+// +build !android
+
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (C) 2017-2018 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+package main
+
+const DoNotUseThisProgramOnLinux = UseTheKernelModuleInstead
+
+// --------------------------------------------------------
+// Do not use this on Linux. Instead use the kernel module.
+// See wireguard.com/install for more information.
+// --------------------------------------------------------