divelist: show/sort nitrox dive oxygen percentage as a range

If you are diving multiple nitrox cylinders, we now show them as a range
instead of just the max.  We'll still sort by max O2 (and for the same
max, by min O2).

So now with trimix dives, we'll show the bottom gas (we assume that
"highest He percentage" is that bottom gas), for nitrox dives we'll show
the range of Oxygen percentage, and for all-air dives we'll show just
"air".

For simple nitrox dives (only a single mix), we'll obviously show just
that single percentage.  This should hopefully conclude the whole "show
multiple cylinders in dive list" mess.

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