udevd: make cgroup logic independent of socket passing

udevd: make cgroup logic independent of socket passing

This should have no behavioural change, but it is odd to tie the cgroup cleaning to
whether or not we are passed sockets.

The point really is if we are guaranteed to be in a dedicated cgroup, so instead
check for our parent being PID1 (we already implicitly only do this on systemd
systems).