Dedicated file logger for netopeer2/netconf

[Krisscut]

Hi !

This is a feature request/proposal.
Are you open to the idea of having a dedicated log file in parallel to the usual console logger to log all the netconf related messages ?
I want to have it separate to the main console output so that I can have both enabled at the same time, but keep the main log at info level for usual operation and the logfile at a more "debug" level to always have the rpc traces available. (I know that it is possible to have the messages in the console output using MSG option)

This is quite useful for instance to see at a glance the content of all the requests/replies transiting through the netopeer2-server.
We can also then parse the file using a tool like https://github.com/AeroFlorian/netconf-parser

On my end I did it patching netopeer2-server (to alter the nc logging callback and create the log file) + the libnetconf itself to filter the display of the get-schema replies. (which are pretty huge)

I attached an example output:
netconf_trace_example.log

And its view in the tool I mentioned:

This is a similar subject as #307

[michalvasko]

No problem, if you create a nice PR, I will merge it. Just make sure the code is clean and all the new features optional.