Wheezy: device removal before Greeter login does not trigger emergency shutdown

Originally created by @anonym on #7333 (Redmine)

From the 1.1~beta1 test session:

When removing Tails USB during Greeter, the emergency shutdown is not triggered, I got a black screen
with “amnesia login:” while removing the same device when Tails is fully started effectively launch
the emergency shutdown

I tested on real hardware and got tons of SQUASHFS errors and no shutdown at all. I confirmed that memlockd and the udev watchdog were running at the time.

Regression since 1.0.

Feature Branch: bugfix/7333-emergency-shutdown-before-login