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/shared/tests.h

@@ -5,9 +5,9 @@
 
 #include "sd-daemon.h"
 
+#include "argv-util.h"
 #include "macro.h"
 #include "static-destruct.h"
-#include "util.h"
 
 static inline bool manager_errno_skip_test(int r) {
         return IN_SET(abs(r),