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

[Adrian Reber]

Hello Andrei,

should I resent without --in-reply-to so that patchwork can correctly
pick it up?

Pavel, what do you think about this latest approach to configuration
files in RPC mode.

I also updated the wiki with a corresponding article:

https://criu.org/Configuration_Files

		Adrian

On Thu, Jun 28, 2018 at 04:02:27PM +0000, Adrian Reber wrote:
> 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.
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