Create core backtrace in unwind hook (do not merge yet)

Related to #829. Requires abrt/satyr#829.

In order to generate core_backtrace from core dump hook, you:
* need to set `UnwindInHook` to `yes` in `/etc/abrt/plugins/CCpp.conf`.
* disable `abrt-action-generate-backtrace` in `post-create` so it won't get overwritten. (We need a configuration option that disables generating full core dumps. I don't think not generating core dumps should be the default behaviour - perhaps we need to discuss this further.)
* need to have kernel 3.18 or later for this feature to work (or backported commit torvalds/linux@b03023ecbdb76c1dec86b41ed80b123c22783220).