Coverity cleanups

[Dirk Hohndel]

As usual before a release I spent a day trying to clean out all their
warnings. A lot of them were clearly false positives, a lot of them were
somewhat questionable but I could see at least the theoretical point and
added some more checks. And a few real bugs were found.

I ignored this last time around (bad me) but this time I tried to clean up
the datatrac.c file which ignored all return values from fread and fseek.
The code now in master should be a lot more robust when given bogus data
files - but I would love it if Salvador could check that this still
actually works. It would be good if we had a test case for this, too.

My plan is still to release Beta 3 tomorrow... as usual, more testing (and
sending of patches / bug fixes) until then is very appreciated.

/D