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-start-unit.c

@@ -306,7 +306,7 @@ int start_unit(int argc, char *argv[], void *userdata) {
                                 /* A command in style of "systemctl start <unit1> <unit2> …", "sysemctl stop <unit1> <unit2> …" and so on */
                                 method = verb_to_method(argv[0]);
                                 job_type = verb_to_job_type(argv[0]);
-                                mode = arg_job_mode;
+                                mode = arg_job_mode();
                         } else
                                 method = job_type = mode = NULL;