Define lots of constants for indexes into the structures for the file

Define lots of constants for indexes into the structures for the file
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.