Enables using range-based "for".

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

-fixes SIGABRT when mpd is started from a previously paused state
-regression introduced by commit: f5a923b9:
'OutputAll: convert to class, move instance to class Partition'

-input_audio_format was previously declared using the static modifier
ensuring it was zero-initialised by default
-the current default-initialised input_audio_format contains garbage at
runtime which allows the AudioFormat.IsDefined() 'fail fast' test in
MultipleOutputs::Update to pass and the SIGABRT follows in
AudioOutput::Open when passed the invalid input_audio_format struct
-switching AudioFormat.IsDefined() for AudioFormat.IsValid() is an
alternative workaround

Has been superseded by the previous commit.

Don't wait for an optimistic write to fail.  This is an improved
workaround for the infamous Raspberry Pi bug (see commit af991765).
It works much better and comes without the negative side effects.  The
old workaround is now obsolete.

There is no advantage in using g_setenv().

Use a listener interface instead of GlobalEvents.

Passing a void pointer is unsafe.

Allows the kernel to combine timer wakeups with other processes,
reducing power usage.

Make it "const".

Another big chunk of code for multi-player support.