Re-raise i3lock when obscured in a separate process

Re-raise i3lock when obscured in a separate process

This is cleaner than commit a305e62 (running authentication in a
separate process) because we don’t lose the feature that X11 will buffer
KeyPresses for us while the process blocks on PAM. See
http://cr.i3wm.org/patch/305 for the amount of code (and new known bugs)
that would have to be introduced to make a305e62 work.