package/cups: Add lp user as default cups user

package/cups: Add lp user as default cups user

This patch is a backport from the rockchip tree.

Author: Jeffy Chen <jeffy.chen@rock-chips.com>
Date:   Tue Nov 13 18:25:34 2018 +0800

    package: cups: Add lp user as default cups user

    Change-Id: Ic7434fe0a7b41b86b5b8b097fa29dd9718e29aa5
    Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
User lp is necessary for running the cups spooler.
Groups lpadmin grants administrative privileges to users.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>