setup: heuristically warn if the FS hosting /etc is not mounted

setup: heuristically warn if the FS hosting /etc is not mounted

Check for the existence of /etc, use -e as it could also be a symlink
(and it's just a heuristic). But only do so if the expected ostype
from the config does not match the detected one, this normally
indicates that we had a "reals" distro running but detected the
fallback "unmanaged". Only warn though, as a hint for the user.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>