This patch diverges quite a bit from the current upstream [1] in a few
ways:
1. virtual_oss(8), virtual_bt_speaker(8) and virtual_oss_cmd(8) are
actually separate programs.
2. Backends (lib/virtual_oss) are built as separate shared libraries and
we dlopen() them in virtual_oss(8) and virtual_bt_speaker(8) on
demand.
3. virtual_equalizer(8) and the sndio and bluetooth backends are built
as ports, because they depend on third-party libraries.
4. Use newer libav API in bluetooth backend (see HAVE_LIBAV ifdefs) to
address compiler errors.
[1] https://github.com/freebsd/virtual_oss
Sponsored by: The FreeBSD Foundation