Disentangle block size from the disk's sector size. Set block size to 1024 to show

Disentangle block size from the disk's sector size.  Set block size to 1024 to show
that they can be different.  Clean up mkfs, simplifying specifying fs parameters,
remove some redundancy between fs and mkfs, and fix disk layout bugs. Call blocks
in the file system blocks instead of sectors.  Passes usertests for different
block sizes.