TS: tested fileno() on EpollReactor / chainability to other (fd) IO-Schedulers.

- Extracted _fdClose, ignore EBADF and EIO, on EINTR retry closing.
  Used instead of os.close()
- EPOLLRDHUP now also as a variable in _epoll_reactor
TS: tracked down 3 fd handling bugs, addTimer now disallows seconds=0

- listdir leaks bugs, workaround using ctypes in test/wl_io/fdutils.py's countFDs()
- fd handling / side-effects when using os.close(<file_like_obj>.fileno()) i.s.o. <file_like_obj>.close():
  ... but the conditions need to be exactly right (find exact cause later)...
    * python's epoll-obj gives EINVAL and/or EBADF's.