For each input thread the callback for "intf-event" was registered
at least twice, and never deleted correctly.
Therefore this patch makes sure that we add the callback only once,
and also properly delete the old one when the input changed.

should close #6207, refs #6252
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>
(cherry picked from commit 53f6bc016209b67a56c2bbecf03ae05c42b7f5ed)