header and central directory structures, and use them as appropriate.
The point being to make it easier to tell what is getting pulled out
where; magic numbers are evil!

Change the computation of the ZipInfo.file_offset field to use the
length of the relevant "extra" field -- there are two different ones,
and the wrong one had been used.  ;-(

This closes SF tracker patch #403276, but more verbosely than the
proposed patch.