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/non-native.yml

@@ -35,6 +35,9 @@ concurrency:
 
 permissions: {}
 
+env:
+  CURL_CI: github
+
 jobs:
   netbsd:
     name: 'NetBSD, CM clang openssl ${{ matrix.arch }}'
@@ -141,6 +144,7 @@ jobs:
           architecture: ${{ matrix.arch }}
           run: |
             export MAKEFLAGS=-j3
+            export CURL_CI=github
             # https://ports.freebsd.org/
             time sudo pkg install -y autoconf automake libtool \
               pkgconf brotli openldap26-client libidn2 libnghttp2 stunnel py311-impacket
@@ -229,6 +233,7 @@ jobs:
             set -e
             ln -s /usr/bin/gcpp /usr/bin/cpp  # Some tests expect `cpp`, which is named `gcpp` in this env.
             export MAKEFLAGS=-j3
+            export CURL_CI=github
             time autoreconf -fi
             mkdir bld && cd bld && time ../configure --enable-unity --enable-test-bundles --enable-debug --enable-warnings --enable-werror \
               --prefix="${HOME}"/install \