Only return events from ppoll that were requested.

Only return events from ppoll that were requested.

If the underlying system's select() returns bits that were not in the
request set, our ppoll() implementation can return revents for events
not requested, which can apparently cause a hang.  Only return revents
for activity in the requested event set.  bz#3416, analysis and fix by
yaroslav.kuzmin at vmssoftware com, ok djm@