Revert "log: fix fallbacks to kmsg"

This reverts commit 4a01181e460686d8b4a543b1dfa7f77c9e3c5ab8.

This patch broke LOG_TARGET_AUTO, i.e. automatic selection of STDERR if
it is a TTY with a fallback on the journal and kmsg otherwise.

The general rule should probably be:

log_open() -- open the "best" possible logging channel according to
log_target configuration.

log_dispatch() -- don't open any log channels ever, with the exception
of kmsg since that has no drawbacks. And do this only on true errors of
the better log channel, not just when it wasn't opened.
man: clarify how to configure default control group hierarchies for services and sessions
update TODO
systemctl: fix indentation