abrt-gdb-exploitable: improve signal analysis by looking at siginfo.si_code

In many cases, we can detect signals which are masquerading
as a segfault, but are really sent via kill(2) et al.

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