[gamers.eclass] fix eclass variable GAME_SCORES
[gamers.eclass] rewrite preserve_scores

* rename to gamers_preserve_vardata
* add -R switch for recursive behavior
* print GAME_SCORES via einfo
* improve documentation and add code examples
* do the copying work in pkg_preinst instead of spreading it over
  pkg_postinst
[gamers.eclass] rewrite dovarlibgames

* rename to gamers_prep_vardata
* don't create non-existing dirs, rather die on it
* add inline documentation
* improve function documentation and add examples
* add einfo for directories being fixed
[games-roguelike/nethack] adjust to new gamers.eclass API