basic: move a bunch of cmdline-related funcs to new argv-util.c+h

I wanted to move saved_arg[cv] to process-util.c+h, but this causes problems:
process-util.h includes format-util.h which includes net/if.h, which conflicts
with linux/if.h. So we can't include process-util.h in some files.

But process-util.c is very long anyway, so it seems nice to create a new file.
rename_process(), invoked_as(), invoked_by_systemd(), and argv_looks_like_help()
which lived in process-util.c refer to saved_argc and saved_argv, so it seems
reasonable to move them to the new file too.

util.c is now empty, so it is removed. util.h remains.

src/basic/argv-util.h

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <stdbool.h>
+
+#include "macro.h"
+
+extern int saved_argc;
+extern char **saved_argv;
+
+static inline void save_argc_argv(int argc, char **argv) {
+        /* Protect against CVE-2021-4034 style attacks */
+        assert_se(argc > 0);
+        assert_se(argv);
+        assert_se(argv[0]);
+
+        saved_argc = argc;
+        saved_argv = argv;
+}
+
+bool invoked_as(char *argv[], const char *token);
+bool invoked_by_systemd(void);
+bool argv_looks_like_help(int argc, char **argv);
+
+int rename_process(const char name[]);