cfg: Log changes to config, not whole config, in cfg_read().

cfg: Log changes to config, not whole config, in cfg_read().

The configuration file is re-read on a regular basis by brcompatd and
vswitchd in practice.  When debug-level logging is enabled on the cfg
module, it was logging the entire config file each time.  Not only is this
a waste of log-file space, it's difficult for humans to see what actually
changed, if anything.

So this commit changes cfg_read() to log a diff instead of the whole config
file.