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_main.c

@@ -53,6 +53,7 @@
 #include "tool_vms.h"
 #include "tool_main.h"
 #include "tool_libinfo.h"
+#include "tool_stderr.h"
 
 /*
  * This is low-level hard-hacking memory leak tracking and similar. Using
@@ -156,7 +157,6 @@ static CURLcode main_init(struct GlobalConfig *config)
 
   /* Initialise the global config */
   config->showerror = FALSE;          /* show errors when silent */
-  config->errors = stderr;            /* Default errors to stderr */
   config->styled_output = TRUE;       /* enable detection */
   config->parallel_max = PARALLEL_DEFAULT;
 
@@ -196,10 +196,6 @@ static void free_globalconfig(struct GlobalConfig *config)
 {
   Curl_safefree(config->trace_dump);
 
-  if(config->errors_fopened && config->errors)
-    fclose(config->errors);
-  config->errors = NULL;
-
   if(config->trace_fopened && config->trace_stream)
     fclose(config->trace_stream);
   config->trace_stream = NULL;
@@ -250,6 +246,8 @@ int main(int argc, char *argv[])
   struct GlobalConfig global;
   memset(&global, 0, sizeof(global));
 
+  tool_init_stderr();
+
 #ifdef WIN32
   /* Undocumented diagnostic option to list the full paths of all loaded
      modules. This is purposely pre-init. */