Stop calling gdk_input_add() with a zero flags word. If we don't

Stop calling gdk_input_add() with a zero flags word. If we don't
want to know about any input events on a socket, it's simpler not to
call gdk_input_add() on it at all.

I hesitate to say `fixes', but ... this change _causes to go away_
the weird problem I had with blank host key dialogs. I have no
understanding of the chain of cause and effect between gdk_input_add
with zero flags and missing redraw events, but it seems like a
change I should make anyway, so I'm going to do so and hope the
problem doesn't come back :-/

[originally from svn r7164]