ao_coreaudio: simplify render callback

Read only the requested amount by the AUHAL (instead of all the buffered data).
No idea what the deal is with pausing the audio units if there is no audio to
play, maybe to avoid underruns of some sort. Anyway from my tests this
condition never occurred so I'm removing it all.
ao_coreaudio: dry up ca_msg and use it everywhere

Change the ca_msg macro to pass along MSGT_AO automatically. Also use it for
every output for consistency.
ao_coreaudio: add error check function and macro

WIP
ao_coreaudio: add error check macro for custom goto labels
ao_coreaudio: rewrite string fetching function
ao_coreaudio: make variable names shorter

property_address -> p_addr
ao_coreaudio: split out some utility functions and refactor them

Split out some utility functions that use the CoreAudio API but are not related
the main task of the AOs (which is to move data correctly to the ringbuffer).
These are mainly need for the verbosity of the CoreAudio API and are just
obscuring the 'real' code.
ao_coreaudio: rewrite spdif render callback
ao_coreaudio: move more functions to _common

Also rewrite them in a *hopefully* clearer way.
ao_coreaudio: simplify digital render callback

It was reported that it also works by not setting the read size in the
AudioBuffer (now idea how, but I will discover it later).
ao_coreaudio: move format listeners to _common and rewrite them
ao_coreaudio: refactor print_help
ao_coreaudio: refactor init function
ao_coreaudio: lpcm: remove buffer size calculation depending on audio unit
ao_coreaudio: split init into init_lpcm and init_digital

By finishing this up I realized there seem to actually be no format
negotiation. If my suspicion is confirmed, that is pretty bad and should be
changed.
ao_coreaudio: move digital related state to struct priv_d
ao_coreaudio: refactor init_digital

This function still needs further work and cleanups. This is a nice step to
commit though.
ao_coreaudio: more cleanups in init_digital
ao_coreaudio: extract methods to lock/unlock device for digital output
ao_coreaudio: move AudioStreamChangeFormat to common file and refactor