New port audio/fossmixer: Graphical mixer application for OSS

FOSS Mixer is a graphical mixer application for FreeBSD to control OSS
audio backend. Mixer can be use to control audio levels, set recording
source, and it includes white noise generator for testing the audio
output. It is pretty much what mixer(1) does. FOSS Mixer does not need
third party user interface library, it runs on plain X11/OpenGL.

PR: 239929
Submitted by: manwe@suomi24.fi