ds: stop caring about event flags set by epoll/poll/kqueue
If we got something to write, then write it. Otherwise, try
reading; and continue dealing with errors which normally occur
along the way.
Trying to read requests while we need to buffer in luserspace
is suicidal from a memory management standpoint.
The only adjustment needed for existing callers is EvCleanup;
where we need to ensure we're always calling the dummy
EvCleanup::event_write callback to accomplish nothing.