Support `$BLOCKSIZE` environment variable

[duckinator]

On the pages for at least df, the standard suggests the use of a BLOCKSIZE environment variable:

The use of 512-byte units is historical practice and maintains compatibility with ls and other utilities in this volume of POSIX.1-2017. This does not mandate that the file system itself be based on 512-byte blocks. The -k option was added as a compromise measure. It was agreed by the standard developers that 512 bytes was the best default unit because of its complete historical consistency on System V (versus the mixed 512/1024-byte usage on BSD systems), and that a -k option to switch to 1024-byte units was a good compromise. Users who prefer the more logical 1024-byte quantity can easily alias df to df -k without breaking many historical scripts relying on the 512-byte units.

It was suggested that df and the various related utilities be modified to access a BLOCKSIZE environment variable to achieve consistency and user acceptance. Since this is not historical practice on any system, it is left as a possible area for system extensions and will be re-evaluated in a future version if it is widely implemented.

I think it's reasonable to only support values of 512 and 1024. To the best of my knowledge, the affected utilities also all have a -k option. Given that information and the way I've implemented Boreutils utililties so far, this seems like a reasonable approach:

// after normal argument parsing is done (to avoid issues if `-k` is passed but $BLOCKSIZE != 1024).
if (!dash_k) { dash_k = strcmp(getenv("BLOCKSIZE"), "1024") == 0; }

The utilities that should have support for this are any which provide information about files and/or has a -k option that switches to 1024 byte blocks. This list should be updated if more are found:

• df
• ls