libfdisk: rename sector_t to fdisk_sector_t

.. just to avoid too generic names for public API.

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: remove 'end' from struct fdisk_partition

This struct member duplicate "start+size".

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: fix bug in cmp_numbers() and partitions sorting

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1170191
Signed-off-by: Karel Zak <kzak@redhat.com>