The zip file header unpack format uses signed longs for 
those values. For the readfile function to work with very 
large files (2GB+ in my case) it needs to use unsigned 
longs. In the case of these files, the readfile code would 
break with a memory error when trying to buffer 
backwards in memory since the value is converted to a 
signed long producing a negative value.

This patch converts the header unpack format to use 
unsigned longs.

-John M. Baughman