Code review

This is my result for code reviewing the debian/postinst file.
It is meant as a sample of work I can immediately contribute to the project: I would like to continue to review the code in devuan, and add comments, to enhance documentation and code readability.
Please forgive me if I accidentaly infringe on others' coding style; I did not find a document establishing a project-wide style.
Actually by ending the sed with g I had got the idea ;)