Use function to get samples to be sent to fill audio buffers to a
level. This replaces the function that only shows how much data
is in the buffer.
This way the function itself can control how much data will be
sent.
Paging tones are not created within sound_alsa.c
The audio API is now responsible to switch to paging channel.
In case of SDR, the sdr.c will switch frequency offset.
Sound instance is now called audio instance and uses funcation pointers.
This gives a clean interface to be exchanged with other technologies,
linke SDR.
Andreas Eversberg