fix: harden security for polkit rules and kauth helper

Kristen McWilliamrequested to merge
work/merritt/kauth-security into master

Addresses several security issues with the polkit rules and kauth helper.

  • Rules default policy is now "no"
  • Rules are further only evaluated if the caller is the plasma-setup user
  • All helpers that accept a username argument check the system for said user, as well as:
    • Getting the user's homedir
    • Denying actions on non-existant users or system users
  • Instead of creating files and folders in the new user's home directory as root and correcting the permissions after the fact, they are created directly as the new user with their permissions

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