fixes for RTS/DTR handling on open (#59)

- internal RTS/DTR state defaults to None
- serialwin32 and serialcli platforms override that to True (as before)
- serialposix leaves it unset and therefore does not touch RTS/DTR on open
  (as it was in pySerial 2.7 and before)
diff --git a/serial/serialutil.py b/serial/serialutil.py
index 6f31efe..eb83b67 100644
--- a/serial/serialutil.py
+++ b/serial/serialutil.py
@@ -152,8 +152,8 @@
         self._dsrdtr = None
         self._inter_byte_timeout = None
         self._rs485_mode = None  # disabled by default
-        self._rts_state = True
-        self._dtr_state = True
+        self._rts_state = None
+        self._dtr_state = None
         self._break_state = False
 
         # assign values using get/set methods using the properties feature