In addition to png.h, configure.ac and CMakeLists.txt, the script
ci_verify_version.sh is now able to verify libpng-config-head.in also.
For the benefit of readability, the old script ci_shellify.sh has been
split into smaller, independent scriptlets: libexec/ci_shellify_*.sh.
The linting script ci_lint.sh has been updated as needed.
As the editorconfig-checker program is transitioning from using
the .ecrc config file to the .editorconfig-checker.json config file,
the older program versions do not recognize the new config file name.
Update ci_lint.sh to instruct editorconfig-checker to pick up its
configuration from the new config file name, regardless of the program
version.
Also update ci_lint.sh to instruct yamllint to check all *.json files,
including .editorconfig-checker.json.
This is a cherry-pick of commit 77f88338a19a223cc678e1a6a04888a31c78dabf
from branch 'libpng18'.
This fixes commit dddaf0c625a8daea4d027cb57380b7fac6f58285.
The way to reliably `find` executable files is different on BSD, Mac
and Linux, unfortunately.
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.
