Doesn't reliably find PulseAudio socket if not in XDG_RUNTIME_DIR

See https://github.com/flatpak/flatpak/issues/4058. Our copy of flatpak_run_add_pulseaudio_args() has the same issue.

We should behave more like libpulse does, implementing the logic that uses ${runtime}/native, where ${runtime} can either be:

• $PULSE_RUNTIME_PATH, or
• $XDG_RUNTIME_DIR/pulse, or
• a temporary directory in /tmp that is pointed to by a symlink ~/.pulse/${machine_id}-runtime or {XDG_CONFIG_HOME:-HOME/.config}/pulse/${machine_id}-runtime
  • where ${machine_id} is either the contents of /etc/machine-id, or the contents of /var/lib/dbus/machine-id, or the hostname

While we're there, we should also contribute this code to Flatpak, resolving flatpak#4058 and maybe flatpak#2169.