tool: use our own stderr variable

Earlier this year we changed our own stderr variable to use the standard
name `stderr` (to avoid bugs where someone is using `stderr` instead of
the curl-tool specific variable). This solution needed to override the
standard `stderr` symbol via the preprocessor. This in turn didn't play
well with unity builds and caused curl tool to crash or stay silent due
to an uninitialized stderr. This was a hard to find issue, fixed by
manually breaking out one file from the unity sources.

To avoid two these two tricks, this patch implements a different
solution: Restore using our own local variable for our stderr output and
leave `stderr` as-is. To avoid using `stderr` by mistake, add a
`checksrc` rule (based on logic we already used in lib for `strerror`)
that detects any `stderr` use in `src` and points to using our own
variable instead: `tool_stderr`.

Follow-up to 06133d3e9b
Follow-up to 2f17a9b654

Closes #11958

src/tool_help.c

@@ -164,7 +164,7 @@ void tool_version_info(void)
 {
   const char *const *builtin;
   if(is_debug())
-    fprintf(stderr, "WARNING: this libcurl is Debug-enabled, "
+    fprintf(tool_stderr, "WARNING: this libcurl is Debug-enabled, "
             "do not use in production\n\n");
 
   printf(CURL_ID "%s\n", curl_version());