nspawn: make use of log_set_open_when_needed() in nspawn too

nspawn: make use of log_set_open_when_needed() in nspawn too

Let's make use of log_set_open_when_needed() in nspawn too, i.e. at the
point where we close logging because we are about to rearrange fds,
let's automatically reopen the logging fds when we need them, the same
way as we do that in the service manager. This makes things simpler and
more robust.