sudo: port over linting & sudoers from admin::sudo

admin::sudo pulls a nifty trick to lint bad sudoers values and
self-heal, as to not let mistakes break sudo for the whole system. Port
it over to sudo::user & sudo::group. While at it, also cleanup our own
sudoers file, basing it off the admin::sudo sudoers (and also fixing the
undefined variable check).

Change-Id: I6fd4dd59242020f7e2b319a52d5a6a6ed9a630cf
admin::sudo: remove privs => [absent] support

Currently unused and complicates the admin/sudo consolidation for no
good reason.

Change-Id: If3a64265107d5818fb32d78ff696081ee7f84fac