Some partitioning schemes want to have partitions that are aligned

Some partitioning schemes want to have partitions that are aligned
with geometry. And they do recalculation of user specified parameters.
MBR, PC98, VTOC8, EBR schemes are doing that. For these schemes an
auto alignment feature (ie. gpart add -a alignment) would not work.
But it can work for GPT and BSD schemes. BSD scheme usualy is created
inside MBR, so we can use knowledge about offset of MBR partition to
calculate aligned values for BSD partitions.

Use "offset" attribute of the parent provider for better alignment.

MFC after:	2 weeks