Rename libarchive write API to "export", matching command line

Rename libarchive write API to "export", matching command line

I was going to add new API for importing, and it was really confusing
that what I think of now as import and export both had "write" in the
name.  It's just clearer to talk about the direction.

At the same time, include `Export` in the options structure.

This isn't an ABI break as the API isn't in a release.