Add ability to configure serial flow control

[thomas-at-rock7]

This commit adds functionality to configure flow control of the serial port. It supports software, hardware or no flow control (the default).

The async_comm::Serial class can configure the flow control in the constructor or calling set_flow_control().

The async_comm::Serial class uses boost::asio::serial_port_base::flow_control::type as async_comm::serial_flow_control.