add --bridge <interface> option to packetblaster lwaftr

[mwiget]

Setting the optional parameter --bridge for packetblaster lwaftr to a linux network interface will bridge non-generator traffic between the selected interface and whatever device packets are blasted over.
The main use case is to allow normal network communications to occur over the interface, while traffic is generated and return traffic measured.

Side use case: if bridging is the only required "feature", then one can set rate to 0 to avoid traffic generation.

Note: it isn't possible to actually communicate with the linux interface directly, but to any other network device connected to the linux interface via external switch.