audio/jamulus: Fix jamulus-server arguments
- allow to override user name for the daemon with jamulus_server_uname
- allow to set server arguments with jamulus_server_args
PR: 260860
Reported by: Stefan Bethke <stb@lassitu.de>
(cherry picked from commit 5e56a22d7125752516dcbf1b89534d38de0bbb9a)