do not wait for fam events indefinitely

when famEventReceived encountered trouble with getting events it'll
disable FAM entirely and instead register another fallback system. this
means we'll have to check if FAM has been disabled every iteration and
if it has break out of the loop. otherwise we'd loop indefinitely.

we'll now also cap the amount of time (or rather iterations) we'll wait
for the event to arrive. this acts as a safety net against further
infinity loops. what's more, since this will get called from the
main thread the GUI is blocked by us, so, 4s is already a lot all
things considered.

FIXED-IN: 5.74
BUG: 423818