decoder: terminate decoder thread before MPD cleanup

When MPD exits, it should manually free all resources in use, to allow
easy memory leak debugging.  Make the decoder thread terminate during
that.

src/decoder_control.c

@@ -100,3 +100,10 @@ dc_seek(struct notify *notify, double where)
 
 	return true;
 }
+
+void
+dc_quit(struct notify *notify)
+{
+	dc.quit = true;
+	dc_command(notify, DECODE_COMMAND_STOP);
+}