Add O_NOCTTY for darwin when opening slave (#93)

Same as in #9, on darwin if there is no controlling terminal then opening a terminal will set that as the controlling terminal for the process which then causes a failure when setting it on the child.

pty_darwin.go

@@ -33,7 +33,7 @@ func open() (pty, tty *os.File, err error) {
 		return nil, nil, err
 	}
 
-	t, err := os.OpenFile(sname, os.O_RDWR, 0)
+	t, err := os.OpenFile(sname, os.O_RDWR|syscall.O_NOCTTY, 0)
 	if err != nil {
 		return nil, nil, err
 	}