doc: ls: support styling and links in --help output

doc: ls: support styling and links in --help output

* src/ls.c (oputs): A new function that wraps puts(),
but also highlights the --option-text portion, and
adds links to the appropriate part of the online manual.
(usage): Call oputs() rather than puts().
* doc/coreutils.texi (--help): Document new HELP_NO_MARKUP env var,
which can be used in the edge case one wants to suppress ansi escapes.
* tests/misc/getopt_vs_usage.sh: Use HELP_NO_MARKUP to ensure the
test continues to pass.