Create new option 'df -b' like *BSD platforms, to explicitly use 512-byte blocks, overriding any BLOCKSIZE specification from the environment.
BSD's options '-P' and '-b' are the same and use 512-byte blocks. However GNU 'df -P' uses 1024-byte block, which makes scripts not portable. I can use 'df -k' which is the same for both but results are inaccurate after conversion.
