unix: do not return non-nil error for 0 errno in FcntlInt

Fixes golang/go#26078

Change-Id: Ie5a8c7028a755bc7a8d56abc4736a5f61ef91ce5
Reviewed-on: https://go-review.googlesource.com/121175
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_unix_test.go

@@ -98,6 +98,24 @@ func TestErrnoSignalName(t *testing.T) {
 	}
 }
 
+func TestFcntlInt(t *testing.T) {
+	t.Parallel()
+	file, err := ioutil.TempFile("", "TestFnctlInt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(file.Name())
+	defer file.Close()
+	f := file.Fd()
+	flags, err := unix.FcntlInt(f, unix.F_GETFD, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if flags&unix.FD_CLOEXEC == 0 {
+		t.Errorf("flags %#x do not include FD_CLOEXEC", flags)
+	}
+}
+
 // TestFcntlFlock tests whether the file locking structure matches
 // the calling convention of each kernel.
 func TestFcntlFlock(t *testing.T) {