nsswitch: do not reload if "/" changes

https://sourceware.org/bugzilla/show_bug.cgi?id=27077



Before reloading nsswitch.conf, verify that the root directory
hasn't changed - if it has, it's likely that we've entered a
container and should not trust the nsswitch inside the container
nor load any shared objects therein.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>