Locking: none.
+ unthrottle(port)
+ Notify the serial driver that characters can now be sent to the serial
+ port without fear of overrunning the input buffers of the line
+ disciplines.
+
+ Locking: none.
+
send_xchar(port,ch)
Transmit a high priority character, even if the port is stopped.
This is used to implement XON/XOFF flow control and tcflow(). If