The old struct config_param remains only for top-level string options.

Make it "const".

Replacing GLib macros.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Keep the device open as long as the output is enabled, but initialize
it only when playback starts.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

Consistent naming.

Move the "extern" declarations from output_list.c, for more type
safety.

Eliminate an unnecessary source of deadlocks.

This is a MPD 0.16 regression: when playing a 24 bit file, the switch
to 16 bit was made only partially, after mBytesPerPacket and
mBytesPerFrame had already been applied.

That means mBytesPerFrame referred to 24 bit, and mBitsPerChannel
referred to 16 bits.  Of course, that cannot work.

Should be safe on OS X 10.4 (32-bit), since Apple's OSStatus boils
down to "signed long", and g_set_error() takes gint, which is really
just "int".  Assigning "signed long" to "int" on 32-bit Unix should be
just fine, since both are signed 32-bit ints.

No idea if this is safe on 64-bit OS X.

Add new config parameter 'device' to audio_output type "osx":
- if not supplied or set to "default", open default device
- if set to "system", open system device
- otherwise 'device' should be an audio device name: mpd will find and
  open the specified audio device, falling back to the default
  device if it's not found

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.