fdisk: refactor /proc/partitions usage

Signed-off-by: Karel Zak <kzak@redhat.com>
sfdisk: add --show-size

Signed-off-by: Karel Zak <kzak@redhat.com>
sfdisk: add comments, remove unnecessary function

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: add fdisk_partition_is_bootable()

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

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