exit() almost there.

- Implemented reasonable way to suspend task.
  - A task that has a pending suspend would be interrupted
    from its sleep via the suspender task.
  - If suspend was raised and right after, task became about to sleep,
    then scheduler wakes it up.
  - If suspend was raised when task was in user mode, then an irq suspends it.
  - Also suspends are checked at the end of a syscall so that if suspend was
    raised because of a syscall from the task, the task is suspended before it
    goes back to user mode.

  - This mechanism is very similar to signals, and it may lead as a base for
    implementing signal handling.

- Implemented common vma dropping for shadow vm object dropping and task exiting.

tasks/test0/src/forktest.c

@@ -32,7 +32,7 @@ int forktest(void)
 	printf("PID: %d, my global: %d\n", myid, global);
 	printf("-- PASSED --\n");
 out:
-	while(1)
-		;
+	printf("PID: %d exiting...\n", myid);
+	_exit(0);
 }