add new run-generator

add new run-generator

This is really useful for running commands like this:

        # systemd-run -i someimage.raw -b systemd.run='"some command line"'

This will now run the command line inside a small Type=oneshot service
and even propagate the exit code of the command back to the parent. And
all that with the full system booted up.

By default this causes the system to shutdown right after the command
completed, but this can be tweaked with systemd.run_success_action= and
systemd.run_failure_action=.

Note that when used in VMs the exit status can of course not be
propagate, as VMs don't really know a concept for that.