purity-ng is a game, put it in /usr/games

[Luke Faraone]

When packaging purity, simonft got:
     E: purity-ng: package-section-games-but-contains-no-game

Purity-ng should be in /usr/games rather than /usr/bin.

[Luke Faraone (lfaraone)]

I'm not sure the best way to do this. "scripts" installs in usr/bin, as suggested by <http://docs.python.org/distutils/setupscript.html#installing-scripts>. The alternative is to specify the path directly, which is sub-optimal.

otoh, our manpage installation is non-portable to non-unix platforms, as is our getch implementation, so perhaps I shouldn't worry about using relative paths that will only work on Linux.