gluon-core: add function to add users

package/gluon-core/files/lib/gluon/functions/users.sh

+add_user() {
+	local username="$1"
+	local id="$2"
+
+	[ "$username" -a "$id" ] || return 1
+
+	sed -i "/^$username:/d" /etc/passwd
+	sed -i "/^$username:/d" /etc/shadow
+
+	echo "$username:*:$id:100:$username:/var:/bin/false" >> /etc/passwd
+	echo "$username:*:0:0:99999:7:::" >> /etc/shadow
+}