audio: Reset audio stream when al_set_audio_stream_playing to false.

Reset stream when al_set_audio_stream_playing is set to false for voice
and mixer cases.  Make all fragments available for refill, resetting the
fake sample position and filling the parts in between fragment buffers
with silence.
audio: Emit stream fragment events when a stream is started.

We usually emit fragment events when a fragment is used up, but if the
stream has zero pending fragments to begin with then no events would
ever be emitted.

An event-driven program might leave the stream unfilled indefinitely -
as I found with ex_audio_chain after the previous change.