INT 21 - DOS 3+ - "FLOCK" - RECORD LOCKING
        AH = 5Ch
        AL = subfunction
            00h lock region of file
            01h unlock region of file
        BX = file handle
        CX:DX = start offset of region within file
        SI:DI = length of region in bytes
Return: CF clear if successful
        CF set on error
            AX = error code (01h,06h,21h,24h) (see AH=59h)
Notes:  error returned unless SHARE or network installed
        an unlock call must specify the same region as some prior lock call
        locked regions become entirely inaccessible to other processes
        duplicate handles created with AH=45h or AH=46h inherit locks, but
          handles inherited by child processes (see AH=4Bh) do not
SeeAlso: AH=BCh,AH=BEh,INT 2F/AX=110Ah,INT 2F/AX=110Bh