launch: Disable autospawn by default when systemd daemon support is enabled.

launch: Disable autospawn by default when systemd daemon support is enabled.

When enabled, this method is prefered over pulseaudio's built in
systems so we should try our best to ensure that it cannot be spawned
outside of the mechanisms desired.

Packagers should call 'systemctl --global enable pulseaudio.socket' to
enable the socket for all users, or alternatively ship an enabling
symlink in /usr/lib/systemd/user/sockets.target.wants/ folder. It may
also make sense for distributions to add in a ConditionNNN= line to the
socket unit if they have a downstream mechanism for enabling or
disabling pulseaudio.

If individual users wish to opt out of this vendor (or administrator)
decision, they can call 'systemctl --user mask pulseaudio.socket'