Take in account the time elapsed since the last callback

Needs testing on device

TimeGet is left as an exercise for tomorrow

this can happen when moving buffers between our 2 linked lists

They stopped working in version 2.0 and never made much sense.

This will replace the "audio-device" variable that was hard-coded into
audio output plugins.

Instead of busy waiting and retrying, buffer packets and wait for the next
pf_play() call to write them to the sound card.
We now have 2 linked lists:
    - packets not accepted yet by Enqueue()
    - packets accepted by Enqueue(), not yet written to hardware, and
    waiting to be released in PlayedCallback()