Make the proxy log into different log files depending on the operation mode.
Make the proxy log into different log files depending on the operation mode.

If the forwarding mode is on, then log into a file that ends with _forward,
otherwise keep the normal log file name.

Fixes https://github.com/netsec-ethz/scion/issues/578

FR4NK-W/osourced-scion,netsec-ethz/scion,klausman/scion,Oncilla/scion,klausman/scion,Oncilla/scion,dmpiergiacomo/scion,dmpiergiacomo/scion,dmpiergiacomo/scion,netsec-ethz/scion,dmpiergiacomo/scion,dmpiergiacomo/scion,FR4NK-W/osourced-scion,klausman/scion,netsec-ethz/scion,dmpiergiacomo/scion,klausman/scion,Oncilla/scion,FR4NK-W/osourced-scion,FR4NK-W/osourced-scion,FR4NK-W/osourced-scion,Oncilla/scion,netsec-ethz/scion,klausman/scion