Eliminate some more getBlockParam() invocations.

This replaces lots of getBlockParam() invocations.

Don't modify the configured string.

Determine the audio output name with audio_output_get_name() instead
of obtaining the name from the configuration again.

Renamed functions, types, variables.

JACK documentation states: "The caller is responsible for calling
free(3) any non-NULL returned value."

This does not seem to include the array elements.  Duplicate them
after jack_get_ports(), and free only the array.  Convert
JackData.output_ports to non-const.

Use GLib's G_GNUC_UNUSED instead of gcc.h's mpd_unused.

Make the pointers "device" and "name" non-const, so we don't need the
xfree() hack.  The default value is expressed as NULL.

Don't return 0/-1 on success/error, but true/false.  Instead of int,
use bool for storing flags.

Don't compile the sources of disabled output plugins at all.

Again, no CamelCase in the directory name.

When the audio source provides 24 bit samples, don't bother to convert
(lossily) them to 16 bit before jack's floating point conversion - go
directly from 24 bit to float.

Move sample format dependent code to a separate function.

Renamed all variables and functions.  Add the prefix "mpd_jack_" to
function names.

We must never pass partial frames.  Added assertions to debug this.

Merge the variables "avail_data" and "avail_frames" into "available".
Both variables are never used at the same time.

The JACK documentation postulates that the process() callback must not
block, therefore locking is forbidden.  Anyway, the old code was racy.

Remove all locks, and don't wait for more data to become available -
just send to the port what is already in the buffer.

Don't wait until there is room for the full data chunk passed to
jack_playAudio().  Try to incrementally send as much as possible into
the ring buffer.

Don't hard-code a frame size of "4" (16 bit stereo), calculate the
sample size from sizeof(*buffer), and create the constant
"frame_size".

Indent with tabs.

The last bit of CamelCase in audio_format.h.  Additionally, rename a
bunch of local variables.

When there are standardized headers, use these instead of the bloated
os_compat.h.

The old struct initializers are error prone and don't allow moving
elements around.  Since we are going to overhaul some of the APIs
soon, it's easier to have all implementations use C99 initializers.

We have eliminated direct accesses to the audio_output struct from
the all output plugins.  Make it opaque for them, and move its real
declaration to output_internal.h, similar to decoder_internal.h.

Pass the opaque structure to plugin.init() only, which will return the
plugin's data pointer on success, and NULL on failure.  This data
pointer will be passed to all other methods instead of the
audio_output struct.

The JACK output plugin needs to reset its "opened" flag when the JACK
server fails.  To prevent it from accessing the audio_output struct
directly introduce the API function audio_output_closed().

Since the output plugin returns a value indicating success or error,
we can have the output core code assign the "open" flag.

Pass the globally configured audio_format as a const pointer to
plugin.init().  plugin.open() gets a writable pointer which contains
the audio_format requested by the plugin.  Its initial value is either
the configured audio_format or the input file's audio_format.

Since the plugin struct is never modified, we should store it in
constant locations.

Also rename AudioOutputPlugin to struct audio_output_plugin, and use
forward declarations to reduce include dependencies.

Just like decoder_api.h, output_api.h provides the audio output API
which is used by the plugins.

Get rid of CamelCase, and don't use a typedef, so we can
forward-declare it, and unclutter the include dependencies.

The previous patch enabled these warnings.  In Eric's branch, they
were worked around with a generic deconst_ptr() function.  There are
several places where we can add "const" to pointers, and in others,
libraries want non-const strings.  In the latter, convert string
literals to "static char[]" variables - this takes the same space, and
seems safer than deconsting a string literal.

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

Storing local configuration in global (static) variables is obviously
a bad idea.  Move all those variables into the JackData struct,
including the locks.

There is only one caller of freeJackData() left: jack_finishDriver().
This function is called by the mpd core, and is called exactly once
for every successful jack_initDriver().  We do not need to clear
audioOutput->data, since this variable is invalidated anyway.

Over the lifetime of the jack AudioOutput object, we want a single
valid JackData object, so we can persistently store data there
(configuration etc.).  Allocate JackData in jack_initDriver().  After
that, we can safely remove all audioOutput->data==NULL checks (and
replace them with assertions).

No need to destroy the JackData object when an error occurs, since
jack_finishDriver() already frees it.  Only deinitialize the jack
library, introduce freeJackClient() for that, and move code from
freeJackData().