decoder/accessor: allow for split input stream variant

decoder/accessor: allow for split input stream variant

When a pxar archive was encoded using the split stream output
variant, access to the payload of regular files has to be redirected
to the corresponding dedicated input.

Allow to pass the split input variant to the decoder and accessor
instances to handle the split streams accordingly and decode split
stream archives.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>