Second patch: rename the internal struct name.  We will eventually
export this type as an opaque forward-declared struct later, so we
can pass a struct pointer instead of a file descriptor, which would
save us an expensive linear lookup.

I don't believe "interface" is a good name for something like
"connection by a client to MPD", let's call it "client".  This is the
first patch in the series which changes the name, beginning with the
file name.

linux/list.h is a nice doubly linked list library - it is lightweight
and powerful at the same time.  It will be useful later, when we begin
to allocate client structures dynamically.  Import it, and strip out
all the stuff which we are not going to use.

The usual bunch of pointer arguments which should be const.

Use "unsigned int" whenever negative values are not meaningful.  Use
size_t whenever we are going to describe buffer sizes.

Only include headers which are really needed.

Give player.c a better name, meaning that the code is used to control
the player thread.

We should avoid having protocol specific code in player.c.  Just
return success or failure, and let the caller send the error code to
the MPD client.

It should be obvious in which thread or context a function is being
executed at runtime.  The code which was left in decode.c is for the
decoder thread itself; give the file a better name.

This is the last of the three variables.  Now we don't need
playerData.h anymore in most sources.

This releases several include file dependencies.  As a side effect,
"CHUNK_SIZE" isn't defined by decoder_api.h anymore, so we have to
define it directly in the plugins which need it.  It just isn't worth
it to add it to the decoder plugin API.

The decoder plugins need this type, so export it in the public API.
This allows is to remove "decode.h" from "decoder_api.h", uncluttering
the API namespace some more.

Now that "dc" is available here, we don't have to pass it to
decoder_is_idle() and decoder_is_starting() anymore.

decode() is a trivial wrapper for decodeParent().  Merge both and
rename them to do_play().

Unfortunately, we have to pass the DecoderControl pointer to these
inline functions, because the global variable "dc" may not be
available here.  This will be fixed later.

When dc->error!=NOERROR, we do not need to check state!=START.
Simplify the checks by moving the error check to the top.

The decoder thread is responsible for resetting dc->command after a
command was executed.  As a consequence, we can assume that
dc->command is already NONE after decoder_stop().

The source "decoder_control.c" provides an API for controlling the
decoder.  This replaces various direct accesses to the DecoderControl
struct.

There is only one caller which passes "true", so we can move the
queueNextSongInPlaylist() invocation there.

There is no unlocked caller of clearPlayerQueue(), and the functions
lockPlaylistInteraction() and unlockPlaylistInteraction() are trivial
- merge them.

Since playerPlay() already calls playerStop(), we can remove its
invocation of playerStop() from playPlaylistOrderNumber().

We can also make playerStop a static function.

There are no nested queue locks in mpd, thus replace the locked checks
in playerQueueLock(), playerQueueUnlock() with assertions.

There is no caller of these two functions which locks the player
queue; replace the playerQueueUnlock() call with an assertion.

All (indirect) callers of queueSong() ensure that the queue state is
BLANK, so there is no need to check it in queueSong() again.  As a
side effect, queueSong() cannot fail anymore, and can return void.
Also, playlist_queueError and all its error handling can go away.

playerKill() was marked as deprecated, but it seems like a good idea
to do proper cleanup in all threads (e.g. for usable valgrind
results).  Introduce the command "EXIT" which makes the player thread
exit cleanly.

playerWait() stops the player thread (twice!) and closes the output
device.  It should be well enough to just send CLOSE_AUDIO, without
STOP.

This requires a tiny change to the player thread code: make it break
when CLOSE_AUDIO is sent.

Again, remove file descriptor parameters, which are not actually
used.  These functions can also be converted to return void.

Most player*() functions do not actually use the file descriptor, and
always return 0 (success).  Eliminate them to get a leaner interface.

It was possible for the decoder thread to go into an endless loop
(flac and oggflac decoders): when a "STOP" command arrived, the Read()
callback would return 0, but the EOF() callback returned false.  Fix:
when decoder_get_command()!=NONE, return EOF==true.

sample_size is a variable which is computed at compile time.  Declare
it "static const", so the compiler can optimize it away.