Encapsulate the horrid gas encoding in gas change events

We should never pass permille values around as integers. And we shouldn't
have to decode the stupid value in more than one place.

This doesn't tackle all the places where we access O2 and He "too early"
and should instead keep passing around a gaxmix. But it's a first step.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

divelist.c

@@ -160,7 +160,7 @@ static int active_o2(struct dive *dive, struct divecomputer *dc, duration_t time
 			break;
 		if (strcmp(event->name, "gaschange"))
 			continue;
-		o2permille = 10 * (event->value & 0xffff);
+		o2permille = get_o2(get_gasmix_from_event(event));
 	}
 	return o2permille;
 }