Unfortunately, the C standard postulates that the argument to free()
must be non-const.  This does not makes sense, and virtually prevents
every pointer which must be freed at some time to be non-const.  Use
the deconst hack (sorry for that) to allow us to free constant
pointers.

Instead of passing the pointer to the "expired" flag to
processListOfCommands(), this function should use the client API to
check this flag.  We can now remove the "global_expired" hack
introduced recently.

Start exporting the client struct as an opaque struct.  For now, pass
it only to processCommand() and processListOfCommands(), and provide a
function to extract the socket handle.  Later, we will propagate the
pointer to all command implementations, and of course to
client_print() etc.

Change the order of function declarations in client.h, to make it well
arranged and readable.

The old code tried to write a response to the client, without even
checking if it was already closed.  Now that we have added more
assertions, these may fail...  perform the "expired" check earlier.

Patch bdeb8e14 ("client: moved "expired" accesses into inline
function") was created under the wrong assumption that
processListOfCommands() could modify the expired flag, which is not
the case.  Although "expired" is a non-const pointer,
processListOfCommands() just reads it, using it as the break condition
in a "while" loop.  I will address this issue with a better overall
solution, but for now provide a pointer to a global "expired" flag.

And again, convert arguments to const.

client_defer_output() was modified so that it can create the
deferred_send list.  With this patch, the assertion on
"deferred_send!=NULL" has become invalid.  Remove it.

I'm really no fan of the warning log, it's too complex
for how little it gets used; but fixing it is another
problem.

Warren hasn't been active in development in a while and probably
doesn't have much time to answer user questions.  So point the
mailing contact to the public mailing lists where any developer
can see and answer.

Why waste 4 bytes for a flag which we can hide in another variable.

Hiding this flag allows us later to remove it easily.

Unclutter the client_new() constructor by moving unrelated complex
code into a separate function.

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.