Prevent finishing greeter by unhandled signals

We have time gap between main() start and KSignalHandler registration
in which signals will close greeter

First this bug tried to fix in commit c63287ca
but placing KSignalHandler registration at start of main() is bad idea
because it broke mechanism for handling signals at all
and this behaviour was fixed in commit 448df751
by moving handlers bellow
for this reason we have this time gap