Add posibility to enable rs485 functionality for port under Linux

[slyshykO]

There is rs485 that provides this functionality.
But it has not been updated for 7 years.
And it is convenient to have such functionality right in place.

[sirhcel]

Thank you for this suggestion! Yes, it would be convenient to support this functionality directly from the serialport crate.

My gut instinct tells me that verifying this functionality will become the hardest part. Do you know of some commodity hardware which we could use for testing? Especially the more exotic stuff like activating a termination resistor?