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_writeout.c

@@ -566,7 +566,7 @@ void ourWriteOut(struct OperationConfig *config, struct per_transfer *per,
                 if(fclose_stream)
                   fclose(stream);
                 fclose_stream = FALSE;
-                stream = stderr;
+                stream = tool_stderr;
                 break;
               case VAR_JSON:
                 ourWriteOutJSON(stream, variables, per, per_result);
@@ -583,7 +583,8 @@ void ourWriteOut(struct OperationConfig *config, struct per_transfer *per,
             }
           }
           if(!match) {
-            fprintf(stderr, "curl: unknown --write-out variable: '%.*s'\n",
+            fprintf(tool_stderr,
+                    "curl: unknown --write-out variable: '%.*s'\n",
                     (int)vlen, ptr);
           }
           ptr = end + 1; /* pass the end */