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/Makefile.inc

@@ -82,6 +82,7 @@ CURL_CFILES = \
   tool_paramhlp.c \
   tool_parsecfg.c \
   tool_progress.c \
+  tool_stderr.c \
   tool_strdup.c \
   tool_setopt.c \
   tool_sleep.c \
@@ -126,6 +127,7 @@ CURL_HFILES = \
   tool_setopt.h \
   tool_setup.h \
   tool_sleep.h \
+  tool_stderr.h \
   tool_strdup.h \
   tool_urlglob.h \
   tool_util.h \