Remove a GLib dependencies from class EventLoop and DatabaseLock.

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.

Track which thread runs the EventLoop and provide a check whether
we're currently inside.

Prepare for adding more code.

Merge with CommitEventFlags().

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.

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.