Support producing multi-fragment zstd archives.

Support producing multi-fragment zstd archives.

When the `zstd:frame-per-file` option is specified, the zstd filter will start a new frame when flushed, i.e. for each file in the archive.

The `zstd:min-frame-size=N` option modifies the `zstd:frame-per-file` option in that it will not start a new frame unless the current one exceeds `N` bytes.

When the `zstd:max-frame-size=N` option is specified, the zstd filter will start a new frame any time the compressed size of the previous one exceeds `N` bytes.

These options decrease compression efficiency by a varying amount (depending on the exact composition of its contents) but render the tarball seekable, to a certain extent.