If no ports are configured, don't overwrite the (NULL) configuration
with the port names of the first JACK server.  If the server changes
after a JACK reconnect, MPD won't attempt to auto-detect again.

Currently, the JACK plugin manipulates the audio_format struct which
was passed to the open() method.  This is very likely to break,
because the plugin must not permanently store this pointer.  After
this patch, MPD ignores sample rate changes.  It looks like other
software is doing the same, and I guess this is a non-issue.

This patch converts the audio_format pointer within jack_data into a
static audio_format struct.

jack_set_info_function() is not provided by older libjack versions.
Attempt to detect if it is available.

Use jack_set_info_function() to install an info callback.  Don't let
libjack print them to stderr.

Return false from mpd_jack_play(), let the MPD core close the device.

Don't leave uninitialized bytes in the jack_data struct.

When MPD stops playback, close the JACK client connection.

The "bps" attribute is calculated, but never used.

Return true/false instead of 1/-1.

Preparation for supporting other channel numbers than stereo: use
loops instead of duplicating code for the second channel.  Most
likely, gcc will unroll these loops, so the binary won't be any
different.

When jack_get_ports() returns NULL, we cannot have any ports to
connect to, and the device cannot play anything.

libjack's jack_port_name() function returns the effective port name,
we don't need to do it manually.

Do the global libjack initialization in the global plugin
initialization function.

When waiting for free space in the ring buffer, the JACK plugin
sleeped 10ms until there is enough space.  This delay was too large
for low-latency setups (<10ms), and created a lot of xruns.  Work
around that by reducing the sleep time to 1ms.

A proper solution for this would be to use an event based approach,
and we will do it, just not now.

When the connection failed once, you had to restart MPD, because it
never cleared the jack_data.shutdown flag.  Instead, it refused to
play anything "because there is no client thread" (which is wrong at
that point).

If the ring buffers are allocated after jack_activate(),
mpd_jack_process() might segfault because it attempts to access them.

Call jack_port_register() before jack_activate().

On some platforms, g_free() must be used for memory allocated by
GLib.  This patch intends to correct a lot of occurrences, but is
probably not complete.

Return the default value in the conf_get_block_*() functions when
param==NULL was passed.

This simplifies a lot of code, because all initialization can be done
in one code path, regardless whether configuration is present.

All config_get_block_*() functions should accept constant config_param
pointers.

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.