Add next cmd

cgpt next will return the partition uuid of the next partition that
should be booted. This can be used in an initrd to figure out what root
partition to use.

It handles decrementing tries so this is a one shot command.

```shell
$ cgpt add -i 4 -T 1 /dev/sda
$ cgpt next
E03DD35C-7C2D-4A47-B3FE-27F15780A57C
$ cgpt next
7130C94A-213A-4E5A-8E26-6CCE9662F132
```