Improve log messages in cccpp hook and save-package-data. See rhbz#909617

If core file can't be opened, say so.
Only for other cases say "it is not a regular file with link count 1".

Properly format absolute coredump names (don't prepend current dir).

Explain why "doesn't belong to any package" message is emitted:
because ProcessUnpackaged isn't set.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>