Feature request: df -b option for 512-byte blocks

[Thomson Blakefield]

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.