benchmarks/linux-unigine-{heaven,valley}: the ports had been improved (+)

• Add missing dependency for playing music: USE_LINUX+=openal-soft
• In the `do-extract', rather than changing current directory, pass it to tar(1) as a -C argument and don't needlessly use semi-private _DISTDIR variable: this makes the command shorter and avoids bogus double slash in the build log
• Provide more user-friendly wrapper script which anticipates potential problems that typically affect running this program and advises on how to solve them
• Spell GNU/Linux correctly in the COMMENT and port description; reformat the latter with `fmt -w 75' for better looks (less ragged right edge)