INT 2F U - DOS 3+ internal - GET JOB FILE TABLE ENTRY
        AX = 1220h
        BX = file handle
Return: CF set on error
            AL = 6 (invalid file handle)
        CF clear if successful
            ES:DI -> JFT entry for file handle in current process
Note:   the byte pointed at by ES:DI contains the number of the SFT for the
          file handle, or FFh if the handle is not open
SeeAlso: AX=1229h