build: delete/replace 3 more clang warning pragmas

- tool_msgs: delete redundant `-Wformat-nonliteral` suppression pragma.

- whitespace formatting in `mprintf.h`, lib518, lib537.

- lib518: fix wrong variable in `sizeof()`.

- lib518: bump variables to `rlim_t`.
  Follow-up to e2b394106d #1469

- lib518: sync error message with lib537
  Follow-up to 365322b8bc

- lib518, lib537: replace `-Wformat-nonliteral` suppression pragmas
  by reworking test code.

Follow-up to 5b286c2508 #12812
Follow-up to aee4ebe591 #12803
Follow-up to 0923012758 #12540
Follow-up to 3829759bd0 #12489

Reviewed-by: Daniel Stenberg
Closes #12814

src/tool_msgs.c

@@ -124,14 +124,7 @@ void helpf(FILE *errors, const char *fmt, ...)
     va_start(ap, fmt);
     DEBUGASSERT(!strchr(fmt, '\n'));
     fputs("curl: ", errors); /* prefix it */
-#ifdef __clang__
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wformat-nonliteral"
-#endif
     vfprintf(errors, fmt, ap);
-#ifdef __clang__
-#pragma clang diagnostic pop
-#endif
     va_end(ap);
     fputs("\n", errors); /* newline it */
   }