config diff: obtain diff between current config and default config

With this we can simply ask the daemon for the diff between what would be
its default and what is its current config.

Current form will output extraneous information that was not directly
supplied by the user though, such as 'host' 'fsid' and 'daemonize', as
well as defaults we may rewrite ourselves (leveldb tunables on the monitor
for instance).  Nonetheless, it's way better than the alternative and
considering it should be used solely for debug purposes I think we can
get away with it.