customize, sysprep: add a short SELinux note

Add a short paragraph about SELinux, mostly to point to the
documentation about it provided in the documentation of virt-builder.
sysprep: add a "changed file" side effect
sysprep: user-account: mark "changed file" if users were removed

Removing an user causes /etc/passwd, /etc/shadow, and /etc/group to
change, so mark the side effects as such if any user has been removed.