Fix broken sudoers files

Lesson learned today: An invalid sudoers file can break the sudo command, which in turn prevent the sudoers file from being edited via sudo. 

The good practice is to always use visudo to modify sudoers file. In my case I needed to modify a file inside /etc/sudoers.d, where I should have used `visudo -f`.

To recover from invalid sudoers files, it is possible to run `pkexec bash` to gain root access. However I got an error "polkit-agent-helper-1: error response to PolicyKit daemon: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: No session for cookie"

Solution to this error:

Source: https://github.com/NixOS/nixpkgs/issues/18012#issuecomment-335350903

- Open two terminals. (tmux also works)

- In terminal #1, get PID by running `echo $$`

- In terminal #2, run `pkttyagent --process <PID>`

- In terminal #1, run `pkexec bash`