Fix various issues with new scripts

The two new scripts (dm-pacman and dmwall) each had two problems:

• They used the /bin/sh shebang, directly violating a rule in the README.md
• They had several issues with shellcheck

This merge request fixes both of these issues as well as minor issues with each script such as typos and inconsistencies.

Please note that neither of these scripts are tested, but my edits shouldn't have broken them. If they have please let me know and I will resubmit with the harmful edits undone.