More notes for myself later.
Started implementing ProxyDataTransferPolicy, for controlling the type of
active/passive data transfer between proxy and backend.  Have it working
for cases where the client sends the PORT command; more work to be done
for the remaining three commands (PASV, EPRT, and EPSV).
Forgot to include the new FTP data transfer header and routines, needed
for implementing the ProxyDataTransferPolicy.