core: read "debug" from kernel commandline and set log level
systemctl: add commands set-default and get-default

systemctl set-default NAME links the default.target to the given unit,
get-default prints out the path to the currently set default target.
systemctl: add command set-log-level

Command changes current log level
systemctl: suggest 'systemctl daemon-reload' without --system

--system is default anyway, and some poor user might type 9
characters without needing to.