ls: add support for explicit file name sorting

ls: add support for explicit file name sorting

Support overriding previous sorting options
with an explicit --sort=name option.

* doc/coreutils.texi (ls invocation): Document the new option.
* src/ls.c (usage): Likewise.
(sort_args): Add the "name" entry, and sort to be consistent
with the ordering presented in --help.
* tests/ls/ls-time.sh: Add test cases.
* NEWS: Mention the new feature.
Suggested by: Tzvetelin Katchov