libfdisk: remove debug dump function, cleanup API

The fdisk_dump_* prefix will be used for sfdisk-like functionality.
The patch also add FDISK_ prefix to fdisk_get_unit() options.

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: basic fdisk_dump_* functions

Signed-off-by: Karel Zak <kzak@redhat.com>