Implement `cksum -l`

According to GNU:
```
       -l, --length=BITS
              digest length in bits; must not exceed the max for the
              blake2 algorithm and must be a multiple of 8
```
Tested in GNU with tests/cksum/cksum-c.sh 
Source:
https://github.com/uutils/coreutils/blob/main/src/uu/cksum/src/cksum.rs