Add some more known details to the MPK unpacker.

[darkstar]

Source for this is the QuickBMS script by aluigi

• The Unknow0x06 field is the file version. Presumably there are version 0, version 1 and version 2 MPK files.
• The version and number of files can be read as signed, since they're cast to signed later anyway.
• Version 0 and 1 files have smaller offset and size fields (32 vs 64 bit)
• The Unknown value for each file is the compression flag. 0x00 is uncompressed, 0x01 (or anything != 0x00?) is compressed.

[mchubby]

Already mentioned in #18, see also:

• https://github.com/vn-tools/arc_unpacker/tree/master/src/dec/mages (GPL v3)
• https://github.com/rdavisau/sg-unpack (MIT)
• https://github.com/CommitteeOfZero/SciAdv.Net/tree/master/src/SciAdvNet.MagesVfs (unknown license)