TODO: Regression test to ensure that a tevent backend can cope with separate read...

TODO: Regression test to ensure that a tevent backend can cope with separate read/write events on a single fd.

This tests the multiplex fd changes to the epoll backend to
ensure they work correctly.

Signed-off-by: Jeremy Allison <jra@samba.org>
TODO: why (fde_wcount - fde_count < 256) ???
      if we get TEVENT_FD_WRITE we should not block
      we should also not read if we don't get TEVENT_FD_READ,
      because this will block