ci: Add more checks to ci_verify_version.sh; refactor

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.
7 files changed