Correctly recognise extended partitions created prior to the addition of sl_uid. Fix tested by okan@.
Correctly recognise extended partitions created prior to the addition of
sl_uid. Fix tested by okan@.

ok deraadt@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars