We don't want normal users on the system to be able to
manipulate plymouth. This will be especially important
later when we run plymouth for post-bootup reasons. This
patch checks peer credentials and sends back a NAK to
clients who aren't root.

plymouthd gets started before the drm modesetting modules are loaded,
so we can't try to access the frame buffer right away.  The plan is to
load the drm modules as soon as possible in the initrd and then tell
plymouthd to put up the splash ASAP.

(Only in text plugin so far)