Issue 429: analyse/lint and format script

[thomasmerz]

This PR fixes #429 by these changes:

  👷 add a fancy GitHub action for 'shellcheck' and 'shfmt'
  👷 moved 'shellcheck' and 'check indentation' to new GitHub Action
  🚨 fix 'shellcheck' warnings
  🚨 fix 'shfmt' warnings

Contrary to my expectations I had to fix some warnings in spectre-meltdown-checker.sh which finalize in some minor changes. 🤷🏻‍♂️

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Please have a look at my latest GitHub Action run in my forked repo:

[thomasmerz]

BTW: I use https://github.com/itspriddle/vim-shellcheck and https://github.com/z0mbix/vim-shfmt for local checking while writing/fixing code and https://github.com/nektos/act for a final local check running in a local GitHub Action in docker containers on my linux machine (which makes some trouble with the docker-in-docker steps on my linux machine 🤷🏻‍♂️ ).

[thomasmerz]

I also added some example GitHub Actions for a bad linting/shell syntax and a good (fixed) one.

[thomasmerz]

Hey @speed47 , can you please review this PR? Thank you.