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.