Tine subsurface updates

[Linus Torvalds]

I committed three small changes to subsurface today:

 - it should now compile again with the latest libdivecomputer (yay!
Another incompatible interface change in LD!)

   Hopefully, it also compiles with older libdivecomputer versions, I
tried to make it auto-detect the latest interface change (that, btw,
is not normally possible, so quite often we've had the situation that
you need to have libdivecomputer match fairly closely in time with
subsurface)

 - I think it should now be safe to change the configuration multiple
times on Windows.

   I don't have a windows box to test on, much less a development
environment. But when I was looking at writing config file updates
(for the final change) I noticed that the windows case seemed unsafe:
we closed (and never re-opened) the windows registry when changing
defaults for units etc. So if you tried to do that twice, it looks
like it would have failed the second time.

 - I made subsurface remember your last choice in dive computer, and
default to that for the next time you try to import something.

   I got really tired of having to choose the same dive computer every
time when I tried to import things many times due to a broken USB
cable.

Somebody should check my windows config changes.

                     Linus