bsdinstall guided partitioning should 4k-align swap and ufs partitions

[Mark Martinec]

Nathan wrote:
> The installer will align all partitions to the GEOM stripesize/offset.
> We could make it do min(4KB, stripesize), but in general I think this is
> better done at the GEOM level.

I doubt any SSD device on the market will want to admit its
internal structure, they all claim 512 sectors AFAICT.
Seems to me the min(4KB,stripesize) would be a safe bet.

  Mark