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

docs/CHECKSRC.md

@@ -137,6 +137,8 @@ Currently these are the extended warnings which can be enabled:
 
 - `STRERROR`: use of banned function strerror()
 
+- `STDERR`: use of banned variable `stderr`
+
 ## Ignore certain warnings
 
 Due to the nature of the source code and the flaws of the `checksrc` tool,