Now that the internal versions of GPT partitions are host-endian,

Now that the internal versions of GPT partitions are host-endian,
simplify PRT_protected_guid() by comparing GUID's with
uuid_compare() rather than converting them to strings and using
strncmp().

Further clarify logic by doing the EFI Sys dance only if
the GUID being tested is not marked as protected.

No intentional functional change.