audio/jack: Option for new OSS driver backend

Add a non-default option to build the new OSS driver backend coming in
the next JACK release. The backend is a complete rewrite based on the
sosso library, with improved stability and low latency capabilities. Let
users preview and test the new backend before it becomes default.
Builds with default options are not affected, thus no PORTREVISION bump.

While here, cleanup two checks in the RC service script, one of them
deprecated, the other was ineffective by mistake.

PR: 280611