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(8): Add a zpoolupgrade rc.d script

If a zpool is created by makefs(8), its version is 5000, i.e., all
feature flags are off. Introduce an rc script to run `zpool upgrade`
over the assigned zpools on the first boot. This is useful to the
ZFS based VM images built from release(7).

Reviewed by: imp, markj, mav
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37282