GRMLBASE/21-usersetup: use adduser for adding user to group

adduser >=3.122 no longer supports adding a user to a group via its
addgroup binary (see
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=664869), so let's
invoke adduser instead.

Closes: #115

etc/grml/fai/config/scripts/GRMLBASE/21-usersetup

@@ -26,7 +26,7 @@ fi
 add_user_to_group() {
    [ -n "$1" ] || return 1
    if grep -q $1 $target/etc/group ; then
-      grep "$1:x:.*$USERNAME" $target/etc/group || $ROOTCMD addgroup $USERNAME $1
+      grep "$1:x:.*$USERNAME" $target/etc/group || $ROOTCMD adduser $USERNAME $1
    fi
 }