src: die during shutdown with everything else

src: die during shutdown with everything else

plymouthd currently avoids getting killed at shutdown.  This causes
filesystems to fail to remount read-only in some cases.

This commit changes things up so that plymouthd dies with everyone else,
but spawns a process to hold open the drm device that can keep the splash
up until the very end.

In order to keep this process alive until the very end, it gets run
from within the initramfs (if available).  This requires adding service
files to jump back into the initramfs at shutdown