• Add SNDIO option and make enabling the sndio backend explicit. It's currently automatically enabled when sndio is available in the build environment.
• Add a OSS default option, so that the OSS backend can be turned off if desired.
• Since there are a lot of audio backend options, group them to make the config dialog more readable.
• USES=localbase:ldflags

PR: 219998
Approved by: mat (mentor), Dan Nelson <dnelson@allantgroup.com> (maintainer)
Differential Revision: https://reviews.freebsd.org/D11227