coredump: when storing an incomplete coredump, add COREDUMP_TRUNCATED=yes

coredump: when storing an incomplete coredump, add COREDUMP_TRUNCATED=yes

We logged about this, but did not attach information directly to the log
entry. It *would* be nice to log the full untruncated size, but afaict, to do
this, we would have to read the full data from the kernel. Doing this just to
log that information seems a bit excessive, in particular when the limit could
be set quite low. So for now let's just add a boolean field.