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...

This patch also cleanups usage of empty topology values (e.g.
MINIMUM_IO_SIZE=0 value should not be returned by NAME=value API. The
binary blkid_topology_get_* is not affected by this change.)

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