Fix install in case SHELLDIR == BINDIR

For some stupid reason the game's launch script had been given the same
file name as the game binary, meaning that anybody who accidentally set
SHELLDIR to the same path as BINDIR when configuring the build would
silently miss out on the launch script and force them to do dumb things
like symlink the game library into system library paths and the like, or
end up with an unplayable game.  This commit gives the launch script a
different file name to the game proper.

I don't often get angry at Daniel Thaler because I understand how much
effort it took to get NitroHack functional, let alone playable, but
making it this easy for potential players to shoot themselves in the
foot is completely inexcusable.