systemctl: add log-level and log-target commands

systemctl: add log-level and log-target commands

This copies the commands log-level and log-target (to query and set the current
settings) from systemd-analyze to systemctl, essentially reverting
a65615ca5d78be0dcd7d9c9b4a663fa75f758606. Controllling the log level settings
of the manager is basic functionality, that should be available even if
systemd-analyze (which is more of an analysis tool) is not installed. This is
like dmesg and journalctl, which should be available even if a debugger and
more advanced tools to analyze the kernel are not available. (Note that dmesg
is used to control the log level too, not just to browse the kernel logs.)

I chose to copy&paste the methods from analyze.c to the new location. There
isn't enough code to share, because acquire_bus() in both places has a
different signature despite the same name, so the only part that is common
is the invocation of sd_bus_set_property().