Motivation for this change

A few things I needed to implement a better syncing strategy without nextcloud-client, but with unison and davfs2:

• Removed myself from the maintainer-list of nextcloud-client as I no longer intend to use this.
• Update davfs2 to 1.6.0 and added a patch to make sure that umount is properly invoked.
• Update unison to 2.51.3.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.