epmp port forwarding enhancements

I have two specific ideas that would expand the port forwarding features on the epmp platform.

1. Allow a port forwarding rule to be specific to an interface, eg. the management interface, especially when it is on a separate managemenet VLAN

2. Allow a port forwarding rule to redirect to a different target port

With both of these features, it would allow for the forwarding of traffic to a device on the LAN side of the CPE's NAT router. For example, when you are providing managed services for the customer's indoor wireless access point, you would easily be able to SSH to or access the web GUI of said device on your management VLAN. Currently, this is possible with UBNT's firmware.