audio/virtual_oss_sndio: New port

Although src commit 9cab9fde5eda ("virtual_oss: Port to base") includes
the sndio backend, we do not hook it to the src build, because it
depends on the third party library libsndio, so we build it as a port
instead.

The port simply builds the files from src, which is not ideal, as this
means that the src repo needs to be present in the system in order for
the port to build, but this way we can avoid version mismatches and
ifdef hell as the project progresses.

Note that this is an extension for the base system virtual_oss, and not
audio/virtual_oss.

Sponsored by: The FreeBSD Foundation
Reviewed by: arrowd
Differential Revision: https://reviews.freebsd.org/D52365