abrt-hook-ccpp: minor refactoring

Related to #829.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Create core backtrace in unwind hook

Related to #829.

We need to implement #882 in order for this to work. This change
requires (yet unreleased) satyr-0.16.

The feature is turned off by default, you need to pass
--enable-dump-time-unwind to configure in order to enable it.

Signed-off-by: Martin Milata <mmilata@redhat.com>