Fulfill missing values in core/frames

File name of __kernel_vsyscall function frame cannot be resolved but we
known that the function comes from kernel.

The frame often appears in backtraces of sleep.

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Fix the compat version of duphash generation

sr_thread_get_duphash() must return NULL for oopses having no reliable
frame.

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Merge branch 'frame_file_names'