into separate append_archive and append_archive_filename functions; the first
takes a "struct archive *" as input, while the second takes a filename, opens
the archive, and calls the first.
There should be no changes in behaviour as a result of this commit; it simply
reorganizes code to make more sense. At some point in the future it may be
possible to share code between append_archive and read_archive, but not yet.
Discussed with: kientzle