tool_operate: override the numeric locale and set "C" by force

Makes curl always use dot as decimal separator for options,
independently of what the locale says. Makes scripts and command lines
portable.

Updated docs accordingly.

Reported-by: Daniel Faust

Fixes #9969
Closes #9972

src/tool_operate.c

@@ -2638,9 +2638,10 @@ CURLcode operate(struct GlobalConfig *global, int argc, argv_item_t argv[])
   CURLcode result = CURLE_OK;
   char *first_arg = argc > 1 ? curlx_convert_tchar_to_UTF8(argv[1]) : NULL;
 
-  /* Setup proper locale from environment */
 #ifdef HAVE_SETLOCALE
+  /* Override locale for number parsing (only) */
   setlocale(LC_ALL, "");
+  setlocale(LC_NUMERIC, "C");
 #endif
 
   /* Parse .curlrc if necessary */