systemctl: use argv[0] not program_invocation_short_name for arg dispatch

The immediate motivation is to allow fuzz-systemctl-parse-argv to cover also
the other code paths. p_i_s_n is not getting set (and it probably shouldn't),
so the fuzzer would only cover the paths for ./systemctl, and not ./reboot,
etc. Looking at argv[0] instead, which is passed as part of the fuzzer data,
fixes that.

But I think in general it's more correct to look at argv[0] here: after all we
have all the information available through local variables and shouldn't go out
of our way to look at a global.