checksrc: avoid extra runs in CI, enable more check locally, fix fallouts

To avoid redundant work in CI and to avoid a single checksrc issue make
all autotools jobs fail. After this patch checksrc issues make fail
the checksrc job, the `dist / verify-out-of-tree-autotools-debug`,
`dist / maketgz-and-verify-in-tree`  jobs and the fuzzer job (if run).
Of these, the `dist` jobs replicate local builds, also testing the build
logic.

Also add a script to check the complete local repository, optionally
with the build tree to verify generated C files.

Also:
- automatically run checksrc in subdirectories having a `checksrc`
  target. (examples, OS400, tests http/client, unit and tunit)
- tests/libtest: make sure to run `checksrc` on generated `lib1521.c`.
  (requires in-tree autotools build.)
- tests: run `checksrc` on targets also for non-`DEBUGBUILD`
  builds. It ensures to check `lib1521.c` in CI via job
  `dist / maketgz-and-verify-in-tree`.
- src: drop redundant `$(builddir)` in autotools builds.
- scripts: add `checksrc-all.sh` script to check all C sources and
  the build directory as an option.
- use the above from CI, also make it verify all generated sources.
- silence `checksrc` issues in generated C sources.
- checksrc: add `-v` option to enable verbose mode.
- checksrc: make verbose mode show checked filename and fix to only
  return error on failure.
- make sure that generated C files pass `checksrc`.

Assisted-by: Daniel Stenberg

Closes #17376

.github/workflows/linux-old.yml

@@ -47,6 +47,7 @@ permissions: {}
 
 env:
   MAKEFLAGS: -j 5
+  CURL_CI: github
   DEBIAN_FRONTEND: noninteractive
 
 jobs: