ReusableBuffer is more generic.

Return the pointer given by PcmBuffer::Get() instead of reaching into
the PcmBuffer object.

Look for "$XDG_CONFIG_HOME/mpd/mpd.conf", similar to where mpd looks
for the file under Windows.

FfmpegDecoderPlugin: Allocate the interleaved buffer as needed instead of using an arbitrary big constant.

Also, save a memcpy when FFmpeg outputs interleaved audio.

Fixes build with FFmpeg 2.0.

Errors are fatal now.  This makes the class a lot easier to use.

Remove the runtime check for eventfd(), hard-code the feature once
it's been selected at compile time.  The class WakeFD is splitted into
EventFD and EventPipe, using WakeFD as a macro diversion.

Don't deinterleave manually, don't allocate memory.

Simplify the error path, because the other allocations may fail.