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_stderr.c

@@ -0,0 +1,72 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+
+/* In this file, stdio.h's stderr macro is not overridden. */
+#define CURL_DO_NOT_OVERRIDE_STDERR
+
+#include "tool_setup.h"
+
+#include "tool_stderr.h"
+
+#include "memdebug.h" /* keep this as LAST include */
+
+/* In other tool files stderr is defined as tool_stderr by tool_setup.h */
+FILE *tool_stderr;
+
+void tool_init_stderr(void)
+{
+  tool_stderr = stderr;
+}
+
+void tool_set_stderr_file(char *filename)
+{
+  FILE *fp;
+
+  if(!filename)
+    return;
+
+  if(!strcmp(filename, "-")) {
+    tool_stderr = stdout;
+    return;
+  }
+
+  /* precheck that filename is accessible to lessen the chance that the
+     subsequent freopen will fail. */
+  fp = fopen(filename, FOPEN_WRITETEXT);
+  if(!fp) {
+    fprintf(tool_stderr, "Warning: Failed to open %s!\n", filename);
+    return;
+  }
+  fclose(fp);
+
+  /* freopen the actual stderr (stdio.h stderr) instead of tool_stderr since
+     the latter may be set to stdout. */
+  fp = freopen(filename, FOPEN_WRITETEXT, stderr);
+  if(!fp) {
+    /* stderr may have been closed by freopen. there is nothing to be done. */
+    DEBUGASSERT(0);
+    return;
+  }
+  tool_stderr = stderr;
+}