function AxMakeComName(const ComNum : Word) : string;
Returns the default device name of a serial port.
AxMakeComName returns an operating system appropriate device name for a given physical serial port number.
For example, if ComNum is 3, AxMakeComName returns "/dev/ttyS2" under Linux.
If ComNum is set to 0, AxMakeComName returns an empty string.
AxMakeComName is interfaced by the AxSystem unit.