unix: add Dup3 on dragonfly

Use the same implementation as on freebsd which is also what the
dragonfly libc uses.

Change-Id: I0ed513ae15fcb6dac77b2fc76f662723d66b75c6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/636435
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

unix/dup3_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build freebsd || linux || netbsd || openbsd
+//go:build dragonfly || freebsd || linux || netbsd || openbsd
 
 package unix_test
 
@@ -16,7 +16,7 @@ import (
 )
 
 func TestDup3(t *testing.T) {
-	tempFile, err := os.Create(filepath.Join(t.TempDir(), "TestDup"))
+	tempFile, err := os.Create(filepath.Join(t.TempDir(), "TestDup3"))
 	if err != nil {
 		t.Fatal(err)
 	}