Major API redesign: don't let the caller allocate the input_stream
object. Let each input plugin allocate its own (derived/extended)
input_stream pointer. The "data" attribute can now be removed, and
all input plugins simply cast the input_stream pointer to their own
structure (with an "struct input_stream base" as the first attribute).
Make the input_stream implementation hold a reference on the
archive_file object. Allow the caller to "close" the archive_file
object immediately, no matter if the open_stream() method has
succeeded or not.
