Make zl-vfs able to load an archive that has had data prepended to it

Comment from physfs, from which this approach was inspired:
>  For self-extracting archives, etc, there's crapola in the file
>  before the zipfile records; we calculate how much data there is
>  prepended by determining how far the central directory offset is
>  from where it is supposed to be (start of end-of-central-dir minus
>  sizeof central dir)...the difference in bytes is how much arbitrary
>  data is at the start of the physical file.
Merge pull request #985 from sclark39/ziploadfix

Make zl-vfs able to load an archive that has had data prepended to it