unix: add Renameat on dragonfly

Also add a test now that all unices have Renameat.

Change-Id: I9098662569e9910122dc686559bbd04be06328fa
Reviewed-on: https://go-review.googlesource.com/c/157898
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_unix_test.go

@@ -623,6 +623,29 @@ func TestMkdev(t *testing.T) {
 	}
 }
 
+func TestRenameat(t *testing.T) {
+	defer chtmpdir(t)()
+
+	from, to := "renamefrom", "renameto"
+
+	touch(t, from)
+
+	err := unix.Renameat(unix.AT_FDCWD, from, unix.AT_FDCWD, to)
+	if err != nil {
+		t.Fatalf("Renameat: unexpected error: %v", err)
+	}
+
+	_, err = os.Stat(to)
+	if err != nil {
+		t.Error(err)
+	}
+
+	_, err = os.Stat(from)
+	if err == nil {
+		t.Errorf("Renameat: stat of renamed file %q unexpectedly suceeded", from)
+	}
+}
+
 // mktmpfifo creates a temporary FIFO and provides a cleanup function.
 func mktmpfifo(t *testing.T) (*os.File, func()) {
 	err := unix.Mkfifo("fifo", 0666)