libblkid: support alignment_offset=-1

libblkid: support alignment_offset=-1

Unfortunately, Linux kernel uses "signed int" for alignment_offset and
the offset could be -1 for devices with undefined alignment (if no
compatible sizes and alignments exist for stacked devices).

There is no way how libblkid caller can respond to the value -1, so
we are going to hide this corner case...

TODO: maybe we can export an extra boolean value 'misaligned' rather
      then complete hide this problem. We will see...

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