ls: add --sort=width option to sort by file name width

ls: add --sort=width option to sort by file name width

This helps identify the outliers for long filenames, and also produces
a more compact display of columns when listing a directory with many
entries of various widths.

* src/ls.c (sort_type, sort_types, sort_width): New sort_width sort
type.
(sort_args): Add "width" sort arg.
(cmp_width, fileinfo_width): New sort function and helper for file name
width.
(quote_name_width): Add function prototype declaration.
(usage): Document --sort=width option.
* doc/coreutils.texi: Document --sort=width option.
* tests/ls/sort-width-option.sh: New test for --sort=width option.
* tests/local.mk: Reference new test.
* NEWS: Mention the new feature.