The variable `CI_LINT_COUNTER` was incremented inside subshells, but
remained unchanged in the main shell process. The errors detected by
the internal linters remained unreported by the main script. (Oopsie!)
Besides fixing this defect, considering that only a pass/fail status
is needed, we are replacing `CI_LINT_COUNTER` with `CI_LINT_STATUS`.
Work around a limitation in the `shellcheck source` directive, which
does not recognize quotes in shellcheck versions older than 0.9.
Also extend the checks for YAML files over the entire source tree, in
preparation for the introduction of the GitHub Actions config file.
