Log Message:
Update to 2.31:

- Uses DatLib v2.21.
- Empty attributes are now ignored (e.g. sha1="")
- Removed cloneof, romof and sampleof fixes from datutil -i
- DatUtil -i, -I and -p now default the output format to CMPro
  (in addition to -g, -G and -r which already behave like that)
- Added support for the isbios="yes" of MAME v0.117u2 onwards.