Rename gt_protected field of struct gpt_type to gt_attr and

Rename gt_protected field of struct gpt_type to gt_attr and
#define GTATTR_PROTECT and GTATTR_PROTECT_EFISYS as bits in it.

Use GTATTR_PROTECT to replace the magic value of '1' indicating
the partition is protected. Use GTATTR_PROTECT_EFISYS to replace
hack of checking for "APFS" string in the names of partitions
that want the EFI System (a.k.a. 0xEF) partition to be preserved.

More flexible and easier to extend protections to new partition
types or add new attributes to partition types.

No intentional functional change.