audio: osx audio fixes

- fix dequeue to return 3 when the requested number of items to dequeue is not the same as the actual number of items. fix comments to reflect this change as well as the actual input and output info for this variable.

- fix comments. fix debug printouts to be consistent.

- fix the AU callback to not throw, since it is not handled robustly by the OS routines that to the call.  Add NOTE to this effect.  Return better error codes when an issue arises.

- correctly convert d_buffer_sample_count to int before using it.

- when stopping: if waiting in ::work, signal the thread to wake up, in the hopes that it will correctly return and terminate.  After stopping the AU, abort and reset buffers as well as clear our local knowledge about the amount of data in the queues.
whitespace only
audio: osx: rename d_buffer_sample_count -> d_buffer_size_samples; truth in naming!
runtime: enable thread priority, same as with Linux; fix header comments to this effect
channels: fixing a minor bug in the flag_fader model.

Bug fix from 'qin 1921' submitted to GR mailing list.
docs: fixed typo with logger docs.
blocks: fixed documentation for what the max_XX block does.
cmake: fixes expansion of ${pyexe_native} for cross builds with the use of VERBATIM.
Merge branch 'maint' of git.gnuradio.org:gnuradio into maint