fix audio multicast artefacts

[cspiel1]

auframe: add util function calc_nsamp()

[sreimers]

I wonder if rem_au.h and something like src/au/util.c is a better place for this.

[cspiel1]

I also thought about this. Then I thought calc_nsamp() has something to do with audio frames. It computes sampc for one audio frame. Anyway, do you prefer util.c?

[sreimers]

It feels more generic to me. At the moment all auframe functions are related to the auframe struct (object). Having a place for generic audio helpers/utilities feels a little better. Maybe we add a function like auframe_nsamp(auframe af, uint16_t ptime) in future.

[cspiel1]

You may still choose. Added a util.c. The last commit could be dropped, or we take it.