[CRIU] Add configuration file support for RPC (v3)

[Adrian Reber]

After our last discussion concerning configuration files in RPC mode I
changed the implementation:

 * Just like in CLI the configuration file is evaluated first and
   can be overwritten via CLI/RPC options.

 * The RPC client has the possibility to say, that configuration files
   should overwrite RPC options.

    req.opts.prefer_config_file = True

   This option can break CRIU if the user adds incompatible options to the
   configuration file.

 * The RPC client can tell CRIU also to ignore any configuration files by
   setting req.opts.no_default_config = False

 * The RPC client can also specify a configuration file in an alternate
   location by setting the environment variable CRIU_CONFIG_FILE.