audio: alternative fix for previous commit

This is a somewhat obscure situation, and happens only if audio starts
again after it has ended (in particular can happens with files where
audio starts later). It doesn't matter much whether audio starts
immediately or some milliseconds later, so simplify it.
command: sub_add with "auto" should not always select a subtitle track

Running "sub_add file.srt auto" during hook execution automatically
selected the first added track. This happened because all tracks added
with sub_add are marked as "external", and external subtitles are always
selected by default.

Fix this by negating the "external" flag when autoselecting subtitles
during loading. The no_default flag exists for this purpose; it was
probably added for libquvi originally, where we had the same issue.