rc.d: add a service jails config to all base system services

This gives more permissions to services (e.g. network access to
services which require this) when they are started as an automatic
service jail.

The sshd patch is important for the sshd-related functionality as
described in the man-page in the service jails part.

The location of the added env vars is supposed to allow overriding them
in rc.conf, and to hard-disable the use of svcj for some parts where it
doesn't make sense or will not work.

Only a subset of all of the services are fully tested (I'm running this
since more than a year with various services started as service jails).
The untested parts should be most of the time ok, in some edge-cases
more permissions are needed inside the service jail.
Differential Revision: https://reviews.freebsd.org/D40371

Remove $FreeBSD$: one-line sh pattern

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

motd: unbreak for source upgrade

In case of source upgrade path from 12.x proper merge of new /etc
installs /etc/motd.template. Becase of that, the system in left
without symlink /etc/motd -> /var/run/motd but with stale /etc/motd contents.

Fix it creating symlink despite of presence of /etc/motd.template.

MFC after: 1 week

/etc/rc.d/motd: Update to accommodate changes in uname(1) and newvers.sh

The recent changes to the uname(1) command removed trailing spaces for
better POSIX conformance, but it broke the regular expression used by
the motd script which expected it. This commit addresses this by removing
the requirement, as it is no longer present.

Additionally, a recent change in newvers.sh introduced a new format for
uname -v, which omited the build number and build dates to improve
reproducible build support. This commit adds support for this new format.

Reported-by: Jamie Landeg-Jones <jamie@catflap.org>
Reviewed-by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D40225

Introduce startmsg and use it in rc scripts

startmsg is a new rc.subr(8) function function to be used instead of
echo(1) when for boot messages. It replaces the often forgotten

check_startmsgs && echo ...

with

startmsg ...

No functional change intended.

I adjusted the commit message and did some final clean-ups of the patch
before committing.

PR: 255207
Reported by: Jose Luis Duran <jlduran@gmail.com>
Reviewed by: imp, 0mp
Approved by: imp (src)
Differential Revision: https://reviews.freebsd.org/D34514

motd: Generate from template to /var/run

Update login(1), its manual pages, similar utilities, and motd.5 to refer to
the new location.

Suggested by: delphij@ (re: r349256)
Reviewed by: bcr (manpages), delphij
Differential Revision: https://reviews.freebsd.org/D20721

rc.d/motd: Update motd more robustly

Use appropriate fsyncs to persist the rewritten /etc/motd file, when a
rewrite is performed.

Reported by: Jonathan Walton <jonathan AT isilon.com>
Reviewed by: allanjude, vangyzen
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D20701

Move the rc framework out of sbin/init into libexec/rc.

The reasons for this are forward looking to pkgbase:
* /sbin/init is a special binary; try not to replace it with
every package update because an rc script was touched.
(a follow-up commit will make init its own package)
* having rc in its own place will allow more easy replacement
of the rc framework with alternatives, such as openrc.

Discussed with: brd (during BSDCam), kmoore
Requested by: cem, bz
PR: 231522
Approved by: re (gjb)