Recommended by our nmh upline, make the W3M option to depend on
www/w3m the default in order to install a text-based browser
to render HTML for nmh. Users can still deselect the option to
obtain a leaner install base. The other HTML rendering options
also still available.

Suggested by: Ken Hornstein <kenh@cmf.nrl.navy.mil> (nmh upline)