libfdisk: add API to disable specified label

Signed-off-by: Karel Zak <kzak@redhat.com>
fdisk: add -t <type> to enable only specified type

In some cases (for example hybrid GPT) it's useful to force fdisk to
read only specified disk label and ignore everything else.

For example:

	# fdisk -l /dev/sda

	Disk /dev/sda: 149.1 GiB, 160041885696 bytes, 312581808 sectors
	Units: sectors of 1 * 512 = 512 bytes
	Sector size (logical/physical): 512 bytes / 512 bytes
	I/O size (minimum/optimal): 512 bytes / 512 bytes
	Disklabel type: gpt
	Disk identifier: 3549290F-417C-4941-8503-F7835109B821

	Device           Start          End   Size Type
	/dev/sda1         2048      2050047  1000M EFI System
	/dev/sda2      2050048      6146047     2G Microsoft basic data
	/dev/sda3      6146048     26462207   9.7G Linux swap
	/dev/sda4     26462208     98142207  34.2G Microsoft basic data
	/dev/sda5     98142208    230662143  63.2G Microsoft basic data
	/dev/sda6    230662144    312580095  39.1G Microsoft basic data

but when GPT is disabled we can access PMBR:

	# fdisk -l -t dos /dev/sda

	Disk /dev/sda: 149.1 GiB, 160041885696 bytes, 312581808 sectors
	Units: sectors of 1 * 512 = 512 bytes
	Sector size (logical/physical): 512 bytes / 512 bytes
	I/O size (minimum/optimal): 512 bytes / 512 bytes
	Disklabel type: dos
	Disk identifier: 0x00000000

	Device    Boot Start       End    Blocks  Id System
	/dev/sda1          1 312581807 156290903+ ee GPT

Reported-by: Craig <util-linux_contact@magister.com.au>
Signed-off-by: Karel Zak <kzak@redhat.com>