add support for io_pgetevents
    
    This is ppoll/pselect equivalent for io_getevents.  It atomically executes
    the following sequence:
    
    	sigset_t origmask;
    
    	pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
    	ret = io_getevents(ctx, min_nr, nr, events, timeout);
    	pthread_sigmask(SIG_SETMASK, &origmask, NULL);
    
    And thus allows to safely mix aio and signals, especially together with
    IO_CMD_POLL.  See the pselect(2) man page for a more detailed explanation.
    
    Signed-off-by: Christoph Hellwig <hch@lst.de>
    [JEM: add sigset size parameter to io_pgetevents]
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>