Gosec revealed security issues - medium and low severity

[minttraveller]

Running gosec ./... reveals about 90 safety or security issues of various severity.

Issues of severity HIGH have been addressed #26 . We should address the rest - fix the true positives and disable the false positives.

In the meantime, the self-test workflow has been temporarily limited to report only issues with security level HIGH or higher.
(see Makefile#L32).

[bruckins]

I started to work on this in the 25-gosec-medium-low branch but this will require quite some work and a better testing. I need to postpone this due to other items of higher priority.