input_stream: return allocated input_stream objects

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).

src/input_plugin.h

@@ -48,8 +48,7 @@ struct input_plugin {
 	 */
 	void (*finish)(void);
 
-	bool (*open)(struct input_stream *is, const char *url,
-		     GError **error_r);
+	struct input_stream *(*open)(const char *uri, GError **error_r);
 	void (*close)(struct input_stream *is);
 
 	struct tag *(*tag)(struct input_stream *is);