Make df -P immune to effects of e.g., the BLOCK_SIZE envvar.

Make df -P immune to effects of e.g., the BLOCK_SIZE envvar.
* NEWS: With -P, the default block size and output format is not
affected by DF_BLOCK_SIZE, BLOCK_SIZE, or BLOCKSIZE.
* src/df.c (main): Implement this.
* doc/coreutils.texi (df invocation): With -P, the default block size
and output format is not affected by DF_BLOCK_SIZE, BLOCK_SIZE, or
BLOCKSIZE.