PASV is a Boolean property that indicates that the
TIdFTPServerThread should listen on a non-standard
DataPort for the FTP DataChannel connection. PASV is used in the command reply handler for the FTP PASV command to create and initialize the DataChannel for the
Authenticated FTP connection, and starts listening for the Client connection on the DataChannel.
PASV is read in the RETR command reply handler to determine if the DataChannel should Connect (when Passive is False) or Listen for the client connection (when Passive is True) prior to transferring FTP data.