MFC r340322-r340324,r340327: periodic/etc/weekly/340.noid

Prevent periodic/etc/weekly/340.noid from descending into root directories
of jails. Jails have their own user/group databases and this script
can produce multiple false warnings, not to mention significant extra
load in case of large jailed subtrees. Leave this check for jailed
invocations of the same script.