INT 2F U - DOS 3+ internal - CLOSE CURRENT FILE
        AX = 1201h
        SS = DOS DS (must be using a DOS internal stack)
        SDA current SFT pointer -> SFT of file to close
Return: CF set on error
        BX???
        CX new reference count of SFT
        ES:DI -> SFT for file
SeeAlso: AX=1227h,INT 21/AH=3Eh