Add #define's for GPT partition attribute bits REQUIRED, IGNORE

Add #define's for GPT partition attribute bits REQUIRED, IGNORE
and BOOTABLE, set BOOTABLE attribute bit instead of using the
incorrect GPTDOSACTIVE value, have 'fdisk -v' print out GPT
partition attributes if any of the 64 bits are set, don't spoof
any partition with REQUIRED bit set.

Prompted by kettenis@ stumbling across a machine with 40+ (!!)
REQUIRED GPT partitions.

Tested & ok kettenis@