core: introduce new RestartForceExitStatus= service setting

This does the inverse of RestartPreventExitStatus=: it forces a restart
of a service when a certain exit status is returned by a service
process.
machined: don't force terminate registered machines

When a machine is registered in machined with CreateMachine it is OK to
kill the machine when it is terminated, but when an existing unit is
simply registered via RegisterMachine we shouldn't do that, as the unit
is controlled by somebody else.
nspawn: when running in a service unit, use systemd for restarts

THis way we can remove cgroup priviliges after setup, but get them back
for the next restart, as we need it.
arch: add crisv32 to uname check
architecture: add tuples for arm