Port Mirroring

Adds support for binding up to two port-mirroring instances to the same Packet Fowarding Engine on MPC3E. This enables to choose multiple mirror destinations by specifying different port-mirroring instances in the filters. One must include the port-mirror-instance instance-name statement at the [edit firewall filter filter-name term term-name then] hierarchy level. One must also include the port-mirror-instance instance-name statement at the [edit chassis fpc number] hierarchy level to specify the FPC to be used.