systemctl: make "systemctl is-system-running" return "offline" if we are not booted with systemd

This sounds like the better place to expose this than in "systemd-notify
--booted".

Also document the so far undocumented "unknown" state the command might
return. And rearrange the table of states documented to be more like the
one for "is-running".

Also, don't document the precise exit code of this function, just say
errors are reported != 0 or > 0...

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