ls: allow --classify to be ignored for non tty output

ls: allow --classify to be ignored for non tty output

Have the `ls` `--classify` option take an optional argument for when to
classify ("always", "auto", "never"), just like the optional argument
for `--color`.  When the optional argument is not specified, default to
"always" for backwards compatibility.

* src/ls.c (usage): Update help text.
(decode_switches): Support an optional argument for --classify.
* tests/ls/classify.sh: Add a new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the new feature.