pi/netsender: allow customisation of config file path with option passed at initialisation of netsender

Currently the config file path is hardcoded. It would be useful if this could be customisable. This would allow for multiple netsender clients to be run on the same machine (assuming the mac is falsified in at least one or both of the config files, given that clashing macs won’t work on the netreceiver/vidgrind side of things).