decoder/Bridge: add flag to make initial seek errors fatal

When the client wants to seek, but the decoder has already finished
decoding the current song, the player restarts the decoder with an
initial seek at the new position.  When this initial seek fails, MPD
pretends nothing has happened and plays this song from the start.

With this new flag, a restarted decoder marks the initial seek as
"essential" and fails the decoder if that seek fails.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/895

src/decoder/Control.hxx

@@ -117,6 +117,12 @@ public:
 
 	bool seek_error;
 	bool seekable;
+
+	/**
+	 * @see #DecoderBridge::initial_seek_essential
+	 */
+	bool initial_seek_essential;
+
 	SongTime seek_time;
 
 private:
@@ -398,11 +404,14 @@ public:
 	 * owned and freed by the decoder
 	 * @param start_time see #DecoderControl
 	 * @param end_time see #DecoderControl
+	 * @param initial_seek_essential see
+	 * #DecoderBridge::initial_seek_essential
 	 * @param pipe the pipe which receives the decoded chunks (owned by
 	 * the caller)
 	 */
 	void Start(std::unique_ptr<DetachedSong> song,
 		   SongTime start_time, SongTime end_time,
+		   bool initial_seek_essential,
 		   MusicBuffer &buffer,
 		   std::shared_ptr<MusicPipe> pipe) noexcept;