sound examples: Simplify audio example

• Merge ossinit.h and basic.c.
• Rename basic.c to audio.c.
• Use err(3) instead of fprintf(3) + exit(3).
• Some style(9) improvements.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D46307