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_cfgable.h

@@ -302,8 +302,6 @@ struct GlobalConfig {
   bool silent;                    /* don't show messages, --silent given */
   bool noprogress;                /* don't show progress bar */
   bool isatty;                    /* Updated internally if output is a tty */
-  FILE *errors;                   /* Error stream, defaults to stderr */
-  bool errors_fopened;            /* Whether error stream isn't stderr */
   char *trace_dump;               /* file to dump the network trace to */
   FILE *trace_stream;
   bool trace_fopened;