fuzz-systemctl-parse-argv: a new fuzzer

Does what the name suggests. Obviously inspired by sudoers, but note that
our tools are not supposed to be installed suid, so there is no privilege
boundary to cross here.

src/systemctl/systemctl.h

@@ -93,3 +93,5 @@ extern char **arg_clean_what;
 extern TimestampStyle arg_timestamp_style;
 extern bool arg_read_only;
 extern bool arg_mkdir;
+
+int systemctl_dispatch_parse_argv(int argc, char *argv[]);