Read zip files that contain only 64-bit header offset, not 64-bit sizes.
Fixes #13367.

Read zip files that contain completely unexpected Extra fields,
provided we do not need to find 64-bit size or header offset information there.
Fixes #13166.

Write zip file entries with 0xFFFFFFFF uncompressed data bytes
correctly (must use zip64 header, since that's the magic indicator).
Fixes new TestZip64EdgeCase. (Noticed while working on the CL.)

Change-Id: I84a22b3995fafab8052b99de8094a9f35a25de5b
Reviewed-on: https://go-review.googlesource.com/18317Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>