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_msgs.c

@@ -54,7 +54,7 @@ static void voutf(struct GlobalConfig *config,
 
     ptr = print_buffer;
     while(len > 0) {
-      fputs(prefix, config->errors);
+      fputs(prefix, stderr);
 
       if(len > width) {
         size_t cut = width-1;
@@ -67,13 +67,13 @@ static void voutf(struct GlobalConfig *config,
              max text width then! */
           cut = width-1;
 
-        (void)fwrite(ptr, cut + 1, 1, config->errors);
-        fputs("\n", config->errors);
+        (void)fwrite(ptr, cut + 1, 1, stderr);
+        fputs("\n", stderr);
         ptr += cut + 1; /* skip the space too */
         len -= cut + 1;
       }
       else {
-        fputs(ptr, config->errors);
+        fputs(ptr, stderr);
         len = 0;
       }
     }