Converts a string from RMS format to file-system (ACP-QIO) format
    or from file-system (ACP-QIO) format to RMS format.

    Format

      SYS$CVT_FILENAME  cvttyp ,srcstr ,inflags ,outbuf ,outlen

                        ,outflags

    C Prototype

      int sys$cvt_filename  (unsigned int cvttyp, void *srcstr,

                            unsigned int inflags, void *outbuf,

                            unsigned short int *outlen, unsigned int

                            *outflags);