New init script doesn't work on ubuntu

[tsg]

The change introduced in #4340 breaks the init script on older ubuntu systems (12.04 and 14.04 reported).

The check for the runuser command doesn't work. This version seems to be correct:

if command -v runuser >/dev/null 2>&1; then
    USER_WRAPPER="runuser"
fi

But even after fixing the above, go-daemon seems to fail when getting root as an option:

failed to switch to user root: No such file or directory

Found by @LeeDr.