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

scripts/checksrc.pl

@@ -50,6 +50,7 @@ my @ignore_line;
 my %warnings_extended = (
     'COPYRIGHTYEAR'    => 'copyright year incorrect',
     'STRERROR',        => 'strerror() detected',
+    'STDERR',          => 'stderr detected',
     );
 
 my %warnings = (
@@ -729,6 +730,18 @@ sub scanfile {
                 }
             }
         }
+        if($warnings{"STDERR"}) {
+            # scan for use of banned stderr. This is not a BANNEDFUNC to
+            # allow for individual enable/disable of this warning.
+            if($l =~ /^([^\"-]*\W)(stderr)[^\"_]/x) {
+                if($1 !~ /^ *\#/) {
+                    # skip preprocessor lines
+                    checkwarn("STDERR",
+                              $line, length($1), $file, $ol,
+                              "use of $2 is banned (use tool_stderr instead)");
+                }
+            }
+        }
         # scan for use of snprintf for curl-internals reasons
         if($l =~ /^(.*\W)(v?snprintf)\s*\(/x) {
             checkwarn("SNPRINTF",