[lipstick] ShutdownScreen: Allow shutdown mode only from privileged sources

Shutdown mode wouldn't be nice to trigger from where ever so let's check
the caller's effective pid/gid to know we should trust them to use it
properly.

Implementation of isPrivileged is grabbed from Password Manager.

Signed-off-by: Kalle Vahlman <kalle.vahlman@movial.com>
Merge pull request #113 from kvahlman/privileged-shutdownmode

[lipstick] ShutdownScreen: Allow shutdown mode only from privileged sources