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_progress.c

@@ -173,7 +173,7 @@ bool progress_meter(struct GlobalConfig *global,
     header = TRUE;
     fputs("DL% UL%  Dled  Uled  Xfers  Live "
           "Total     Current  Left    Speed\n",
-          global->errors);
+          stderr);
   }
   if(final || (diff > 500)) {
     char time_left[10];
@@ -275,7 +275,7 @@ bool progress_meter(struct GlobalConfig *global,
     }
     time2str(time_spent, spent);
 
-    fprintf(global->errors,
+    fprintf(stderr,
             "\r"
             "%-3s " /* percent downloaded */
             "%-3s " /* percent uploaded */