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

Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.

Make all uses of MusicChunk safe.

Instead of passing whole chunks to the MusicPipe and checking the
end_time after each chunk, truncate the last chunk if it would exceed
the end_time.  This requires keeping track of the absolute PCM frame
number.

This fixes a problem with gapless CUE song transitions: a small part
of the following song was always played twice.

Closes #113

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.