[ftp] Cope with RETR completion prior to all data received

[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.