systemctl: unset const char* arguments in static destructors

When fuzzing, the following happens:
- we parse 'data' and produce an argv array,
- one of the items in argv is assigned to arg_host,
- the argv array is subsequently freed by strv_freep(), and arg_host has a dangling symlink.

In normal use, argv is static, so arg_host can never become a dangling pointer.
In fuzz-systemctl-parse-argv, if we repeatedly parse the same array, we
have some dangling pointers while we're in the middle of parsing. If we parse
the same array a second time, at the end all the dangling pointers will have been
replaced again. But for a short time, if parsing one of the arguments uses another
argument, we would use a dangling pointer.

Such a case occurs when we have --host=… --boot-loader-entry=help. The latter calls
acquire_bus() which uses arg_host.

I'm not particularly happy with making the code more complicated just for
fuzzing, but I think it's better to resolve this, even if the issue cannot
occur in normal invocations, than to deal with fuzzer reports.

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=31714.

src/systemctl/systemctl-kill.c

@@ -22,7 +22,7 @@ int kill_unit(int argc, char *argv[], void *userdata) {
                 arg_kill_who = "all";
 
         /* --fail was specified */
-        if (streq(arg_job_mode, "fail"))
+        if (streq(arg_job_mode(), "fail"))
                 kill_who = strjoina(arg_kill_who, "-fail");
 
         r = expand_unit_names(bus, strv_skip(argv, 1), NULL, &names, NULL);