While we do not recommend the idiom ".Fl Fl long" for long options

While we do not recommend the idiom ".Fl Fl long" for long options
because it is an abuse of semantic macros for device-specific
presentational effects, this idiom is so widespread that it makes
sense to convert it to the recommended ".Fl \-long" during the
validation phase.  For example, this improves HTML formatting
in pages where authors have used the dubious .Fl Fl.

Feature suggested by Steffen Nurpmeso <steffen at sdaoden dot eu>
on freebsd-hackers.