zm.rxd_header_pos (for convenience).
Bug-fixes to zmodem_send_from():
*  ZCRCW subpackets mark the end of frame, so a new ZDATA header must be sent
   after the ZACK is received from the receiver.
*  The ZACK is no longer accepted if it's for the wrong offset.
*  The back-channel traffic check was for ACK rather than ZACK (from zmrx.c)
*  Added more "is_connected" checking in loops.