MEDIUM: startup: warn when chroot is not set for root

MEDIUM: startup: warn when chroot is not set for root

We're still regularly seeing insecure configs where chroot is missing.
Now that we have "chroot auto", there's no excuse for not knowing where
to chroot, so let's detect that we're starting as root, detect that the
process is allowed to chroot (i.e. no capability issue, or some hardened
containers), and if no chroot is set, let's emit a warning explaining how
to silence it, i.e. either "chroot auto" or "chroot /".

Most likely we'll start using "chroot auto" by default in 3.5 if no
usability issue is reported.