added flag "decoder.seeking"

This flag is used internally; it is set by decoder_seek_where(), and
indicates that the decoder plugin has begun the seek process.  It is
used for the case that the decoder plugin has to read data during the
seek process.  Before this patch, that was impossible, because
decoder_read() would refuse to read data unless dc->command is NONE.
This patch is kind of a dirty workaround, and needs to be redesigned
later.

src/decoder_internal.h

@@ -26,6 +26,8 @@ struct decoder {
 	struct decoder_plugin *plugin;
 
 	ConvState conv_state;
+
+	int seeking;
 };
 
 #endif