df: env var with a zero block size should be ignored #8918
Description
If one of the block size environment variables (DF_BLOCK_SIZE, BLOCK_SIZE, or BLOCKSIZE) is zero, our numerical output consists of inf and NaN:
$ DF_BLOCK_SIZE=0 cargo run -q df --output=size,used
0B-blocks Used
inf NaN
inf inf
inf inf
inf inf
inf inf
inf inf
inf NaN
inf inf
inf inf
GNU df, on the other hand, uses the default block size in such a case:
$ DF_BLOCK_SIZE=0 df --output=size,used
1K-blocks Used
3941200 0
4007640 1544
88 53
458020024 190675612
4007640 562952
4007640 28380
1024 0
262144 262144
801528 228