Replace Bsnprintf() implementation and add format attributes

[arogge]

This PR replaces the implementation of Bvsnprintf() by the plain library functions.
We also add the attributes to all printf()-like functions to be able to detect problems at compile-time instead of crashing at runtime. As this produced a lot of problems, -Werror was disabled for format-related things (but should be re-enabled before this PR is merged).

Please check

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If you have any questions or problems, please give a comment in the PR.

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• Git Workflow
• Automatic Sourcecode Formatting
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Checklist for the reviewer of the PR (will be processed by the Bareos team)

Make sure you check/merge the PR using devtools/pr-tool to have some simple automated checks run and a proper changelog record added.

General

• Is the PR title usable as CHANGELOG entry?
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• Check backport line
• Required backport PRs have been created

Source code quality

• Source code changes are understandable
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• the temporary commit disabling format errors has been removed

Tests

• Decision taken that a test is required (if not, then remove this paragraph)
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[sebsura]

I set this to no build to reduce the strain on jenkins. As soon as we get it to build locally this should be changed back.