Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

Allow compiling test programs with only selected plugins.

Replace InputPlugin attributes.

Replaces the method Close().

Merge some duplicate code.

Decouple some more from GLib.

Replaces GLib's GError.

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

Move code from the Vorbis decoder plugin.

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.