Handling RTS with an UART that doesn't directly drives the RTS pin

[Bernard Fouché]

It took me some time to figure that putc/get/set_config etc. are per 
hardware driver type, not per UART... hence only one 'hook' for any of 
these function for a given driver is available, and when the hook 
function is called it has trouble to figure what UART is concerned since 
it can't know about the 'priv' field setup by the underlying driver. I 
found a hack to know what UART is concerned but this isn't very clean... 
no good solution yet!