Implementation of a global variable where all ports that are currently in use are stored.
Ports are removed from the list as soon as they are released.
3 functions are implemented: CanBeUsed, AddtoPortList, RemovePortFromList.
CanBeUsed: checks if port is in the list and returns true or false.
AddtoPortList: checks if port is already in the list and adds it if it is not.
RemovePortFromList: removes port from list when its use has finished.