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.
The entire comment block should be rewritten, perhaps along the lines of:
```shell
# Create /etc/apt/sources.list.d/devuan.list during the configure stage
#
# This script will make a best-effort attempt at guessing the needed suite and
# dist values, falling back to sane defaults should the attempt fail.
# No changes will be made if the file already exists.
```