decoder_control: removed the global variable "dc"

Allocate a decoder_control object where needed, and pass it around.
This will allow more than one decoder thread one day.

src/decoder_internal.h

@@ -26,6 +26,8 @@
 struct input_stream;
 
 struct decoder {
+	struct decoder_control *dc;
+
 	struct pcm_convert_state conv_state;
 
 	bool seeking;