Make the dive gas record the single highest mix

.. using the regular sorting rules: sort by Helium content first, Oxygen
content second.  Air always sorts last (even behind the theoretical
hypoxic Nitrox that nobody sane would use).

This is what Don Kinney implies would be the natural thing for a trimix
diver.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>