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

rc: Set var_run_enable to enable by default

This will load/save the /var/run directories at boot/shutdown if
and only if /var/run/ is a tmpfs mount so it is a win for tmpfs
users and a no-op for everyone else.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D44097
Sponsored by: Beckhoff Automation GmbH & Co. KG

rc.d/var_run: Add needed "shutdown" keyword

The "shutdown" keyword invokes rcorder with the -k flag, for rc scripts
with the keyword at shutdown.

Reported by: bdrewery
Fixes: 27b9777c28b4
MFC after: 3 days
Differential Revision:

libexec/rc: Add var_run rc script

Users with a tmpfs /var/run will lose the directory tree state of
/var/run at reboot. This rc script will optionally (by default)
capture the state of the directory structure in /var/run prior to
shutdown and recreate it at system boot.

Alternatively a user can save the state of the /var/run directories
manually using service var_run save and disable the autosaving of
/var/run state using the var_run_autosave variable, for those
paranoid SSD users.

PR: 259585, 259699
Reported by: freebsd@walstatt-de.de,
Reviewed by: philip, gbe (previous version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36386