ls: fix too few display columns edge case

ls: fix too few display columns edge case

In the edge case where the right most column would
consist of only files taking 1 or 2 cells, we accounted
for 3 cells, which would result in using 1 column too few.
This should have been part of commit v8.24-59-ge71be1292

* src/ls.c (init_column_info): Don't account for separator
in the the final column.
* tests/ls/w-option.sh: Add a test case.