nixos/davfs2: wrap {,u}mount.davfs with setuid=true

[ju1m]

Motivation for this change

Enabling davfs2 and setting a davfs mount point in /etc/fstab:

services.davfs2.enable = true;
fileSystems."/home/julm/mnt/example/julm" = {
  device = "https://nextcloud.example.org/remote.php/dav/files/julm/";
  fsType = "davfs";
  options =
    let conf = pkgs.writeText "davfs2.conf" ''
      backup_dir /home/julm/.backup/davfs2/example/julm
      cache_dir /home/julm/.cache/davfs2/example/julm
    ''; in
    [ "conf=${conf}" "user" "noexec" "nosuid" "noauto" ];
};

Currently leads to:

$ mount ~/mnt/example/julm
/run/current-system/sw/bin/mount.davfs: program is not setuid root

This PR makes mount.davfs and umount.davfs setuid root to make it work.
Note also that fba4c99 (mentioned in #103683) is required for umount to also work.

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.

[flokli]

This seems reasonable, and what Debian does as well:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=573241