systemd-shutdown: use log_set_prohibit_ipc(true)

systemd-shutdown: use log_set_prohibit_ipc(true)

Now we have log_set_prohibit_ipc(), let's use it to clarify that
systemd-shutdown is not expected to try and log via journald (which it is
about to kill).  We avoided ever asking systemd-shutdown to do this, but
it's more convenient for the reader if they don't have to think about that.
In that sense, it's similar to using assert() to validate a function's
arguments.