Function add_fcheck

fn add_fcheck(cmf: u8, flg: u8) -> u8

Generate FCHECK from CMF and FLG (without FCKECH )so that they are correct according to the specification, i.e (CMF*256 + FCHK) % 31 = 0. Returns flg with the FCHKECK bits added (any existing FCHECK bits are ignored).