ls: exclude newline from consideration in line width

ls: exclude newline from consideration in line width

* src/ls.c (calculate_columns): Treat width as inclusive max.
(print_with_separator): Likewise.  Also handle commas explicitly,
since they're not catered for by an implicit newline in the count.
* tests/ls/w-option.sh: Adjust exact-fit column tests.
Note this change also makes the existing `ls -w4 -x -T0 a b` test
behave consistently with other output width limits.
Also add the test case from:
https://github.com/coreutils/coreutils/pull/213
* tests/ls/m-option.sh: Add a test case to ensure appropriate
wrapping when trailing comma at the line limit.
* NEWS: Mention the change in behavior.