util: Fine tune running_in_chroot() a bit

util: Fine tune running_in_chroot() a bit

Let's be a bit more careful when detecting chroot() environments, so that we
can discern them from namespaced environments.

Previously this would simply check if the root directory of PID 1 matches our
own root directory. With this commit, we also check whether the namespaces of
PID 1 and ourselves are the same. If not we assume we are running inside of a
namespaced environment instead of a chroot() environment.

This has the benefit that systemctl (which uses running_in_chroot()) will work
as usual when invoked in a namespaced service.