tinc 1.1: missing PONG

[Todd C. Miller]

Great.  I've created https://github.com/gsliepen/tinc/pull/183
I decided to increase the generation number on io_add() as well as
io_del().  This also removes the need to make a local copy of
event_count since we'll exit the loop if an event is added.

BTW, I don't know if you saw my comment on commit
e8a60109fc91a42420ec626b63956771675f89b0 but that change
leads to a corrupted stack on Windows.

 - todd