do_fork(): RLIMIT_NPROC check

1. mcexec sets RLIMIT_NPROC to the number of mcexec threads.
2. do_fork() gets the current number of threads by calling rusage function.
3. do_fork() returns -EAGAIN when the limit is exceeded.

executer/user/mcexec.c

@@ -1956,6 +1956,10 @@ int main(int argc, char **argv)
 		n_threads = ncpu;
 	}
 
+	/* Quick fix for #900: Limit the number of clone() */
+	desc->rlimit[MCK_RLIMIT_NPROC].rlim_cur = n_threads;
+	desc->rlimit[MCK_RLIMIT_NPROC].rlim_max = n_threads;
+
 	/* 
 	 * XXX: keep thread_data ncpu sized despite that there are only
 	 * n_threads worker threads in the pool so that signaling code