audio/sndio: Various fixes

• Success in opening /dev/dsp* O_RDWR does not mean the device supports play/recording. Add an extra check to see if the requested mode is really supported.
• sndiod cannot currently be started in jails as root, because it tries to setpriority(-20). Allow setpriority to fail and start the daemon anyway.
• Fallback to /dev/dsp instead of /dev/dsp0 when using libsndio directly without a running sndiod.
• Implement sio_setvol in the OSS backend for application level volume control with just libsndio.

PR: 216571
Submitted by: Tobias Kortkamp <t@tobik.me> (maintainer)