Move the gasmix cleanups from XML parsing to the generic dive fixup stage

Right now we do certain cylinder info operations only when importing
from an XML file, which is wrong.  In particular, we do the "is the
gasmix air" or "what is the standard cylinder name" only at XML read
time, which means that if you import a dive directly from the dive
computer, it won't have the air sanitization or the proper default
cylinder names.

Of course, most dive computers don't actually save enough cylinder
information for us to do the cylinder name lookup anyway, but some do.
And all Nitrox-capable dive computers do have that O2 percentage that
needs cleanup too.

Reported-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>