Add 'dar' to 'Backups'

[per2jensen]

Thank you for taking the time to work on a PR for Awesome-Sysadmin!

To ensure your PR is dealt with swiftly please check the following:

• Your additions are Free software
• Software your are submitting is not your own, unless you have a healthy ecosystem with a few contributors (which aren't your sock puppet accounts).
• Submit one item per pull request. This eases reviewing and speeds up inclusion.
• Dar - dar which stands for Disk ARchive, is a robust and rich featured archiving and backup software of the tar style. Source Code, GPLv2 C++
• Additions are inserted preserving alphabetical order.
• Additions are not already listed at awesome-selfhosted
• The Language tag is the main server-side requirement for the software. Don't include frameworks or specific dialects.
• You have searched the repository for any relevant issues or PRs, including closed ones.
• Any category you are creating has the minimum requirement of 3 items.
• Any software project you are adding to the list is actively maintained.
• The pull request title is informative, unlike "Update README.md".
  Suggested titles: "Add aaa to bbb" for adding software aaa to section bbb,
  "Remove aaa from bbb" for removing, "Fix license for aaa", etc.

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Why it is awesome

'dar' is much more robust than tar, the maintainer is diligent on fixing errors. 'dar' provides a lot of functionality in a small package and is simple to operate in a restore scenario in the (possibly distant) future.

[Technetium1]

Could add tags: GPLv2 C++