sudo: adjust sudoers for compat with newer sudo

As NEWS.Debian informs us:
- sudo >= 1.7.4p4-2 is resetting $HOME and $MAIL to the target user when
  sudoing. Undo this for $HOME by adding it to env_keep as we're too
  used to it.
- sudo >= 1.8.2-1 is not configured with --with-secure-path anymore and
  the setting has been moved into a sudoers directive. Add this
  directive to our own sudoers file.

Finally, remove the special-handling of the "sudo" group. We don't use
this here and we shouldn't treat it like that, especially since we don't
manage the group members.

Change-Id: I9052a89b4aa4a534fe2ca4473aa15aa72ad3d76b