Fix LESS environment variable setup in OptionParser#help_exit

[kyanagi]

If the original value of LESS ends with an option starting with --, simply appending Fe would result in an invalid option string.

% LESS=--RAW-CONTROL-CHARS ruby -roptparse -e 'OptionParser.new.parse(ARGV)' -- --help
There is no RAW-CONTROL-CHARSFe option ("less --help" for help)
Press RETURN to continue

As far as I have tested, it seems that LESS can safely contain preceding spaces.

[fzakaria]

I just hit this as I use the following in my Nix configuration:

  home.sessionVariables = {
    LESS = "--quit-if-one-screen --RAW-CONTROL-CHARS";
    EDITOR = "vim";
    # TODO(fmzakari): I might want this if I use home-manager on Linux non-NixOS
    # https://nixos.org/manual/nixpkgs/stable/#locales
    # LOCALE_ARCHIVE = "${pkgs.glibcLocales}/lib/locale/locale-archive";
  };

Thanks for opening the issue; it saved me some hair pulling.