Update udev file to use dedicated "g60cuse" user

Since this user has the ability to create new character devices under /dev
and implement unrestricted ioctls, let's limit it to a dedicated service
account.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
README: Add missing pkg-config dependency for Ubuntu

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>