audio/portaudio: Update to 19.6.0

• Sanitize version a bit. The version scheme is all over the place on other systems [1]. Upstream calls it 19.6.0 in the release notes so do the same in the port.

[1] https://repology.org/metapackage/portaudio/versions

Changes: https://app.assembla.com/wiki/show/portaudio/pa_stable_v190600_20161030
ABI: https://abi-laboratory.pro/tracker/timeline/portaudio/
PR: 229851
Submitted by: ndowens.fbsd@yandex.com