BUG10994: SNAT rules are missing the outgoing interface

BUG10994: SNAT rules are missing the outgoing interface

When creating SNAT rules, the outgoing interface is not set. As a side
effect, traffic that should be send unnatted to a vpn tunnel can be
natted which is a BUG.
With this patch the SNAT rules are getting a outgoing interface
according to the configuration. When selecting the RED Target network,
all SNAT rules will be configured with "-o red0". Otherwise if "all" is
selected, there is no interface in the rule, which matches all networks.

Signed-off-by: Alexander Marx <alexander.marx@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>