(#2053273) Let SysV compat. shutdown commands honor inhibitors for root

[dtardon]

Resolves: #2053273

[dtardon]

@msekletar It just occurred to me that there's an easier--and safer--way than trying to make the new code backwards-compatible again. Given that the changes are in a single function halt_main, we could add the whole function under a new name and then call the either old one or the new one as needed. Something like:

static int halt_main(void) {
        if (env_var_is_set())
                return halt_main_new();
        else
                return halt_main_old();
}

[msekletar]

That sounds much better. Please try to implement it this way so we know how exactly would resulting code look.

[dtardon]

Done.

[msekletar]

LGTM