ls: allow -w0 to mean no limit on line length
* src/ls.c (print_with_separator): Renamed from print_with_commas,
and parameterized to accept the separator to print.
Also fix an edge case where '\n' not printed when
the POS variable overflows SIZE_MAX.
(print_current_files): Degenerate -x and -C to using the
cheaper print_with_separator() in the -w0 case.
* doc/coreutils.texi (ls invocation): Document the new feature.
* tests/ls/w-option.sh: A new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the improvement.
Fixes http://bugs.gnu.org/21325