[ftp] Cope with RETR completion prior to all data received
Based on a patch contributed by Sergey Vlasov <vsu@altlinux.ru> :
In my testing with "qemu -net user" the 226 response to RETR was
often received earlier than final packets of the data connection;
this caused the received file to become truncated without any error
indication. Fix this by adding an intermediate state FTP_TRANSFER
between FTP_RETR and FTP_QUIT, so that the transfer is considered to
be complete only when both the end of data connection is encountered
and the final reply to the RETR command is received.