fdisk: cleanup alignment, default to 1MiB offset

fdisk: cleanup alignment, default to 1MiB offset

* add heuristic to detect that the device does not provide
  topology information. We can use topology if:

	- alignment_offset is not 0
	- or optimal_io_size is not 0
	- or minimal_io_size is not power of 2

* default to 1MiB offset for the start of the first partition
  if topology is unknown. This (2048 512-byte sectors)
  is Windows Vista default.

* use optimal_io_size, minimal_io_size or 1MiB as a grain for
  partitions alignment

Note that this all is used when DOS-compatible mode is disabled.

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