Handle ppoll, pselect, epoll_pwait atomicity for race conditions

The above system calls behave as if this code was atomic in the kernel

```
sigprocmask(.., &new_mask, &old_mask);
select();
sigprocmask(.., &old_mask, null);
```

DR (patch TB committed shortly) will emulate the system calls in terms of switching out the sigmasks, but will not prevent race conditions as the system call intends.