POSIXify padd.sh

[yubiuser]

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

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What does this PR aim to accomplish?:
Make padd.sh posix compliant.

[yubiuser]

Ready for review. Quiz: which is /bash and /sh ;-)

[dschaper]

That's so awesome! Thanks!

[yubiuser]

Rebased on latest development, merge conflicts resolved.