ao_coreaudio: remove packetSize private variable
ao_coreaudio: change private vars names to match mpv conventions
ao_coreaudio: remove TODO

AudioDeviceAddIOProc is deprecated! AudioDeviceCreateIOProcID is the new way
to do this.
ao_coreaudio: change all ++var to var++

Luckily they all were inside for loops so the functionality does not actually
change.
ao_coreaudio: print help string in one go
ao_coreaudio: remove device property listener on uninit

Also extract this functionality inside a function in coreaudio_common