tool: improve --stderr handling

- freopen stderr with the user-specified file (--stderr file) instead of
  using a separate 'errors' stream.

- In tool_setup.h override stdio.h's stderr macro as global variable
  tool_stderr.

Both freopen and overriding the stderr macro are necessary because if
the user-specified filename is "-" then stdout is assigned to
tool_stderr and no freopen takes place. See the PR for more information.

Ref: https://github.com/curl/curl/issues/10491

Closes https://github.com/curl/curl/pull/10673

src/tool_writeout.c

@@ -427,7 +427,7 @@ void ourWriteOut(struct OperationConfig *config, struct per_transfer *per,
                 stream = stdout;
                 break;
               case VAR_STDERR:
-                stream = config->global->errors;
+                stream = stderr;
                 break;
               case VAR_JSON:
                 ourWriteOutJSON(stream, variables, per, per_result);