gpt: add commands to manipulate pMBR boot flag

gpt: add commands to manipulate pMBR boot flag

Some BIOS systems will only boot from GPT partitions if the boot flag is
set on the protective MBR partition. This adds the ability to set this
flag using the disk_set and disk_toggle commands.
* include/parted/disk.in.h (_PedDiskFlag): Add PED_DISK_GPT_PMBR_BOOT
* libparted/disk.c (ped_disk_flag_get_name): Add PED_DISK_GPT_PMBR_BOOT
* libparted/labels/gpt.c (_GPTDiskData): Add pmbr_boot flag.
(gpt_alloc): Init pmbr_boot to 0.
(gpt_read_headers): Set pmbr_boot state from PMBR boot flag.
(_write_pmbr): Add pmbr_boot flag and set PMBR boot flag from it.
(gpt_write): Pass pmbr_boot flag through to _write_pmbr
(gpt_disk_set_flag): New function
(gpt_disk_is_flag_available): New function
(gpt_disk_get_flag): New function
(gpt_disk_ops): Add disk_set_flag, disk_get_flag, disk_is_flag_available
* parted/parted.c (do_disk_set): New function
(do_disk_toggle): New function
(_init_commands): Add do_disk_set and do_disk_toggle
* parted/ui.c (command_line_get_disk_flag): New function
* parted/ui.h: Add command_line_get_disk_flag prototype.