systemctl: move set-log-level to systemd-analyze

"systemctl set-log-level" is a command for analysis and tracing hence
"systemd-analyze" should be the better home for it, thus allowing us to
make the overly large "systemctl" a bit smaller.
systemctl: remove "load" command

"systemctl load" has always been racy since the GC could hit any time,
before making use of the loaded unit. Very recent systemd will run GC
immeidately after all unit state changes which has the effect that the
the effect of "systemctl load" is completely gone now, so let's remove
the support for it in "systemctl" for good.