Avoid wasting 4 kB stack per directory level.

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

Replaces GLib's GError.

Add the interface ArchiveVisitor.

Eliminate input_stream_init() and input_stream_deinit().

Hide the definition from C code, to prepare the transition to C++.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.