FTAM — listening socket determination of source / destination File

In general (exception noted below) it is not possible for a receiving
FileTransferSession to know what is the destination File for the data
coming in over a socket. The receiver is never made aware of the
sender's connection host and port, so it cannot distinguish multiple
connections to the listening socket.

The exception is that the transfer mechanism described in Chapter 4
will work reliably as long as the following limitations apply:

1. A FileTransferSession is involved in only one file transfer at a
time so there is no ambiguity as to the data source and destination.

AND

2. A server process implementation makes each FileTransferSession
listen on a different port or allow only one FileTransferSession to
take part in a transfer at a time. This guarantees there is no
ambiguity as to which FileTransferSession is involved in a file
transfer.

These are rather severe restrictions. Simultaneous transfers and reuse
of server sockets can be accomplished if the transfer negotiation is
augmented to exchange details such as host and port (somewhat like the
ftp PORT command does to establish a data connection.) Such an approach
is outlined in issue 4209.

The resolution for issues 4177, 4209, and 4227 applies to this issue.