Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

periodic/weekly/340.noid: do not use sysrc(8) that depends
on bsdinstall(8) and may be non-functional
for system built WITHOUT_BSDINSTALL. (*)

Also, add a check for jails sharing whole tree with host (path=/)
and do not skip it.

Reported by: Andre.Albsmeier@siemens.com (*)
MFC after: 1 week

weekly/340.noid: suppress warning on non-existent jail.conf

MFC after: 1 month

Sigh... Fix another breakage after r340322: spell "sysrc" correctly.

MFC after: 1 month

Fix breakage after r340322: add missing "-n" to invocation of sysrc.

MFH: 1 month

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.

MFC after: 1 month

Move all periodic related config and scripts to usr.sbin/periodic/

This makes pkgbase easier by tagging these as CONFS so they are properly
tagged as config files.

Approved by: will (mentor)
Sponsored by: Essen Hackathon
Differential Revision: https://reviews.freebsd.org/D16553