zip: first cut at writing zip64.

This automatically handles the more-than-64k-files cases.

The large file case is trickier and not fully handled right now.  The
problem is that we need to know in advance if the length will be larger
than 4G or, alternatively, we must unconditionally use zip64 format.
The problem is, we don't know if our consumers can handle it.  No
released version of libgsf to date can handle it, for example.