os: Make OsSignalHandler ask for core dumps for signo != SIGQUIT

SIGQUIT is a normal termination request, but any other signal we handle
here wants a core. This has the effect of making FatalError's call to
AbortServer trigger the

    if (CoreDump)
        OsAbort();

path. This will allow us to remove some DDX code that has the same net
effect.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>