audio/rsynth: Fix audio output

[tobik]

Summary

- Add Sndio backend from [1] and fix it
  - Remove dead code
  - Fix format selection and playback with raw devices
- Fix OSS backend
  - Remove dead code
  - Don't open the device in non-blocking mode.  There is no error
    handling for short writes and looking at how audio_play() is used in say.c
    it is expected to block.
  - Fix format selection
  - Let the kernel handle sample conversion instead of doing it in the backend
- Make NAS support optional
- Make installing dictionary database tools optional
- Add LICENSE

Obtained from:	OpenBSD [1] (based on)
Approved by:	??? (mentor)
Differential Revision:	https://reviews.freebsd.org/D?????

Test Plan

poudriere testport 11.0/amd64 ok