virtual_oss: Port to base

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
MFC after: 1 week
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D52308

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