Make it work for 32 bits platform with _FILE_OFFSET_BITS=64

[jief666]

There was a arithmetic problem on 32 bits platform when off_t is 64 bits.
Basically, we have to make sure to convert to 64 bits BEFORE the multiplication. Hence things like : ((off_t)extent->startBlock) * blockSize.

[jief666]

After this one, if you're still interested in that project, I can submit few pull requests.
I worked on :
removing memory leaks
removing warning
extended attributes
sparsebundle, clear or encrypted, support
adding fuse support
UTF8 encoding
As of today, mounting an image with libdmg is the exact same as mounting it on Mac. Test with huge images 300-400TB on Mac and on linux 32 bits.