Fix crash condition where kscreenlocker_greet doesn't start handle signals in time

Devin Lin requested to merge work/devinlin/fiximmediatecrash into master

On mobile, we frequently spam our power buttons, of which each triggers a call from PowerDevil to ksldapp (in ksmserver) to lock the screen.

On the first call, it will start the greeter process. Subsequent calls send SIGUSR1 to the greeter process which is expected to be handled.

However, the greeter process doesn't start handling signals until after the lockscreen has loaded, which can take a few seconds.

During this period, any attempts to lock the screen (ex. pressing the power button) will cause the lockscreen to crash since SIGUSR1 isn't handled.

This MR ensures signal handling starts immediately at greeter start.

Fixes teams/plasma-mobile/issues#187 (closed)

Edited by Devin Lin

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