Add three GPT attributes:

GPT_ENT_ATTR_BOOTME - this is bootable partition

GPT_ENT_ATTR_BOOTONCE - try to boot only once from this partition

GPT_ENT_ATTR_BOOTFAILED - set this flag if we cannot boot from partition
containing GPT_ENT_ATTR_BOOTONCE flag; note that if we cannot
boot from partition that contains only GPT_ENT_ATTR_BOOTME flag,
the GPT_ENT_ATTR_BOOTFAILED flag won't be set

According to wikipedia Microsoft TechNet says that attributes are divided into
two halves: the lower 4 bytes representing partition independent attributes,
and the upper 4 bytes are partition type dependent. Microsoft is already using
bits 60 (read-only), 62 (hidden) and 63 (do not automount) and I'd like to not
collide with those, so we are using bit 59 (bootme), 58 (bootonce) and 57
(bootfailed).

Reviewed by: arch (Message-ID: <20100917234542.GE1902@garage.freebsd.pl>)
MFC after: 2 weeks