core: make sure that $JOURNAL_STREAM prefers stderr over stdout information (#6824)

core: make sure that $JOURNAL_STREAM prefers stderr over stdout information (#6824)

If two separate log streams are connected to stdout and stderr, let's
make sure $JOURNAL_STREAM points to the latter, as that's the preferred
log destination, and the environment variable has been created in order
to permit services to automatically upgrade from stderr based logging to
native journal logging.

Also, document this behaviour.

Fixes: #6800