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

Add zfskeys rc.d script for auto-loading encryption keys

ZFS in 13 supports encryption, but for the use case where keys are
available in plaintext on disk there is no mechanism for automatically
loading keys on startup.

This script will, by default, look for any dataset with encryption and
keylocation prefixed with file://. It will attempt to unlock, timing
out after 10 seconds for each dataset found.
User can optionally specify explicitly which datasets to attempt to
unlock.

Also supports (optionally by force) unmounting filesystems and unloading
associated keys.

Sponsored by: Modirum
Differential Revision: https://reviews.freebsd.org/D30015