archive: replaced setup_stream() with open_stream()

The open_stream() method opens the input_stream.  This allows the
archive plugin to do its own initialization, and it also allows it to
use input_stream.data.  We can remove input_stream.archive now, which
was unnatural to have in the first place.

src/input_stream.h

@@ -77,8 +77,6 @@ struct input_stream {
 	 * the MIME content type of the resource, or NULL if unknown
 	 */
 	char *mime;
-
-	void *archive;
 };
 
 /**