Copy the reason_phrase from the SoupMessage, and check for transport
errors in the session callback.

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.

To check for errors without reading.  The decoder thread wants to do
that, before it passes the input stream to the plugin.

Fix memory leak.

libsoup's asynchronous API is not thread safe.  By moving the calls
into the I/O thread, several crash bugs will be fixed.

Add the "complete" attribute, and set it in _session_callback().
_close() waits for it to become true.

Add attribute postponed_error, pass this GError to the caller.

Fix deadlock.

We'll add proper locking now, and there's no need to carry this
duplicate flag.

To report failures to callers who don't use _buffer().

Avoid recursive deadlock in _session_callback().

To demonstrate the new I/O thread.  libsoup is well-integrated into
the GLib main loop, which made this plugin pretty easy to write.

As a side effect, we have to initialize the I/O thread in all debug
programs that use the input API.