The last patch removed the "continue" directive, and now the while
loop is without function.  Remove it.  Also make client_manager_io()
return 0.

Previously, when select() failed, we assumed that there was an invalid
file descriptor in one of the client structs.  Thus we tried select()
one by one.  This is bogus, because we should never have invalid file
descriptors.  Remove it, and make select() errors fatal.

Eliminate duplicated code, call client_defer_output() which we
splitted from client_write_output() earlier.

Move the second part of client_write_output() into a separate
function.

client_defer_output() was designed to add new buffers to an existing
deferred_send buffer.  Tweak it and allow it to create a new buffer
list.

Exit the function when an error occurs, and move the rest of the
following code one indent level left.

Split the large function client_write_output() into two parts; this is
the first code moving patch.

All of the client's resources are freed in client_close().  It is
enough to set the "expired" flag, no need to duplicate lots of
destruction code again and again.

Due to the large buffers in the client struct, the static client array
eats several megabytes of RAM with a maximum of only 10 clients.  Stop
this waste and allocate each client struct from the heap.

The code becomes less complex and more readable when we move this
linear search into a separate mini function.

This saves one level of indent.

Functions which operate on the whole client list are prefixed with
"client_manager_", and functions which handle just one client just get
"client_".

Rename all static functions, variables and macros which have
"interface" in their name to something nicer prefixed with "client_".

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.