Ref #46 - altered flush() for posx to use tcdrain rather than tcflush

[euantorano]

Fixes #46 - Rather than using tcflush, use tcdrain.

The descriptions for the two functions are as follows:

• tcdrain:

  waits until all output written to the object referred to by fd has been transmitted

• tcflush:

  discards data written to the object referred to by fd but not transmitted, or data received but not read, depending on the value of queue_selector

[euantorano]

Note: This has only been tested on my Mac, sending to a Windows laptop. I need to dig out some other machines to test on.

I also want to double check all the other flags we set, compared to pyserial: https://github.com/pyserial/pyserial/blob/master/serial/serialposix.py#L400