The port's type of output flow control. 'none'
indicates no flow control at this level or below.
'xonXoff' indicates software flow control by
recognizing XON and XOFF characters. 'hardware'
indicates flow control delegated to the lower level,
for example a parallel port.
'ctsRts' and 'dsrDtr' are specific to RS-232-like
ports. Although not architecturally pure, they are
included here for simplicy's sake.