core: list iptables sharing rules in the right order

core: list iptables sharing rules in the right order

The rules were added to the list using g_slist_append() and then
applied one at time using "iptables --insert" which puts them at the
beginning of the chain, reversing the initial order.

Instead, list them in the desired order and use g_slist_prepend() to
achieve the same result. This has no functional changes.