Fix shellcheck issues in Kea scripts

@ondrej pointed out that shellcheck, a tool that sanitizes shell scripts, reported issues in keactrl.

This task is about addressing those issues in keactrl and checking other user-visible scripts.

The tool is available as on-line service (https://www.shellcheck.net/), but also as command line tool.

Moved this directly to 1.6, because it's coming from Debian package maintainer, so it's related to an objective of 1.6.