generators: only redirect logging when invoked by systemd

generators: only redirect logging when invoked by systemd

We would always print output to the kmsg or journal, but that is only needed
and useful when invoked by systemd. So let's skip redirection unless we are
invoked by systemd. Otherwise, let's log normally. This makes test invocations
easier, and also helps when the generator is invoked by mistake. If redirection
is necessary, the generator can be invoked with SYSTEMD_LOG_TARGET=… even
during tests.