Fix maxdepth calculation on replan

[Dirk Hohndel]

On Mon, Nov 03, 2014 at 10:42:34PM +0100, Robert Helling wrote:
> Hi,
> 
> the maximal depth of a dive was not correctly calculated when replanting made it shallower.

Sadly your attached patches are always "octed-stream" instead of "text",
so it's hard to include your patch in a quote...

I don't like this patch as it removes a feature. We have seen a few dive
computers that store a maximum dive depth in the dive header, but when you
go through the samples, none of the samples are that deep. Our conclusion
was that the dive computer stores the absolute maximum depth that was
reached during a dive (and not just the deepest depth at any of the
sampling points). So we really don't want to just ignore the maximum depth
in the dc structure... for our artificial dive computer structures that we
use in planning we should simply manually clear that value before calling
fixup_dive_dc - but we should NOT clear this for dives downloaded from a
dive computer.

/D