aubuf: add AUBUF_FILE mode

[cspiel1]

Dropping old frames on first read is not practicable for the file play
use-case.

[sreimers]

Is a new mode necessary? ab->wish_sz/min_sz = 0 should do the same right?

[cspiel1]

A positive wish_sz might also help to achieve a stable start of a file playback.
The play.c in baresip can be simplified if we introduce the AUBUF_FILE mode. Concretely, the minsz stuff can be removed from play.c:
baresip/baresip@0ece152#diff-b2feb29ef04159a46a64332fe2dc2f064fb4167d9623c87f2c82109038f1e321L350

[sreimers]

Maybe a more generic mode name like AUBUF_STREAM is better (and file reading/writing as a use case example comment)?

[cspiel1]

Maybe a more generic mode name like AUBUF_STREAM is better (and file reading/writing as a use case example comment)?

It is stream vs file. But again was thinking about this. We should not mix the mode with this feature. The last commit adds a "live" flag. This was inspired by gstreamers "is-live" flag in element audiotestsrc.

[cspiel1]

Thanks for review and merging this!
This one is related: baresip/baresip#2363