Prevent finishing greeter by unhandled signals

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

On mobile platform action for screen lock may generated very often and sometime ksld send SIGUSR1 during this gap

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

Solution - block needed signals on main() start and unblock after handlers registration