settings section in XML file and trips issue

[Dirk Hohndel]

I pushed two commits to master.

The first continues the divecomputer nickname work. I went with
<divecomputerid> for the tag name in the settings section (seemed
clearer then having the same tag mean different things and have
different semantics depending on where it is in the tree).

Please test that this works for you. It should now do the smart thing
for people with multiple divecomputers (and be a complete no-op for all
the normal people with only one).

The second commit fixes a crash that is caused by some serious issues in
the trip handling code. Linus, you have rewritten this quite heavily, so
maybe you want to look at it?

Simply import both your and my dives with DEBUG_TRIP defined and you'll
see things go wrong big time.

Yes, merging dives from two different divers that were on the same dive
trips is not really a well defined concept to begin with. But at least
we should come up with rules that keep our data structures in a sane
state...

/D