JULABO: enable xon flow control by default

A julabo may be in hardware or software flow control. If in software and the ioc does not know then yo see `<13>` and `<11>` (Ctrl-S / Ctrl-Q) characters being sent from the julabo and causing the ioc to parse error. I believe it wouldbe safe to just enable xon by default. In asyn

ixon controls XON/OFF flow control on output. If the IOC receives an XOFF character, it suspends output until an XON character is received.

ixoff controls XON/OFF flow control on input. The IOC sends XOFF and XON characters as necessary to prevent input from coming in faster than programs are reading it. The external device sending the input data must respond to an XOFF character by suspending transmission, and to an XON character by resuming transmission.

So if we just enable `ixon` but not `ixoff` it should respond to the characters if sent from the julabo, but never send them to the julabo. There is a minor chance some data corruption might accidentally get interpreted as an `xoff`, do we think that is an issue? Serial parameters use `even` parity so there is some basic error checking.

For reference see

https://github.com/ISISComputingGroup/IBEX/issues/3865
https://github.com/ISISComputingGroup/IBEX/issues/3971