Add vendor

Specifically because I want to modify a dependency

vendor/golang.org/x/sys/unix/sockcmsg_dragonfly.go

@@ -0,0 +1,16 @@
+// Copyright 2019 The Go 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 unix
+
+// Round the length of a raw sockaddr up to align it properly.
+func cmsgAlignOf(salen int) int {
+	salign := SizeofPtr
+	if SizeofPtr == 8 && !supportsABI(_dragonflyABIChangeVersion) {
+		// 64-bit Dragonfly before the September 2019 ABI changes still requires
+		// 32-bit aligned access to network subsystem.
+		salign = 4
+	}
+	return (salen + salign - 1) & ^(salign - 1)
+}