Read ZIP64 extended information

The extra block of a file header can contain a ZIP64 that overrides some
of the fields from the file header with longer values if the fields had
been set to 0xffffffff (or 0xffff for the disk number).

See sections 4.5.2, 4.5.3, 4.4.8, 4.4.9, 4.4.13 and 4.4.16 of the
specification for reference.

Closes cthackers/adm-zip#81 as fixed.