wipe now obsessively tries to write that last short block.
Also: comments!

git-svn-id: svn://graphite/diminuto/trunk@491 5dfbf39a-1452-0410-8a97-9e58fedde8e3
Added sector size and made blocksize a multiple of that.

git-svn-id: svn://graphite/diminuto/trunk@492 5dfbf39a-1452-0410-8a97-9e58fedde8e3