protect against forksbombs and friends in runguard

fix-runguard.diff

diff --git a/judge/runguard.c b/judge/runguard.c
index 84d7f40..2cb28c1 100644
--- a/judge/runguard.c
+++ b/judge/runguard.c
@@ -462,6 +462,21 @@ void cgroup_attach()
        cgroup_free(&cg);
 }

+void cgroup_kill()
+{
+       int ret;
+       void *handle = NULL;
+       pid_t pid;
+
+       /* kill any remaining tasks, and wait for them to be gone */
+       while(1) {
+               ret = cgroup_get_task_begin(cgroupname, "memory", &handle, &pid);
+               cgroup_get_task_end(&handle);
+               if (ret == ECGEOF) break;
+               kill(pid, SIGKILL);
+       }
+}
+
 void cgroup_delete()
 {
        int ret;
@@ -1151,6 +1166,7 @@ int main(int argc, char **argv)

 #ifdef USE_CGROUPS
                output_cgroup_stats();
+               cgroup_kill();
                cgroup_delete();
 #endif