[CRIU] Configuration files in master

[Adrian Reber]

Veronika and I discussed the missing things to get configuration file
support merged into master.

The easy points are:

 * test was added with 4e68d555829e6095e878cc4f15892f5442651fd6 and runs
   during every zdtm run

 * man page has a whole section called 'CONFIGURATION FILES'

 * wiki can easily be added based on the man page

The difficult part is RPC and Veronika and I do not agree totally how to
handle it and we are hoping for some input.


The first question is if RPC should read configuration files at all.
Looking at the ticket https://github.com/checkpoint-restore/criu/issues/278
I would say that it is what user would expect. But the details get a bit
complicated.

On the command-line the configuration file can be overrule by the CLI.

If the configuration file has 'tcp-established' I can tell CRIU with
--no-tcp-established to ignore the value from the configuration file.
How should this be handled in the RPC case. My expectation would be that
the configuration file can overwrite the settings from RPC. That can
lead to strange behaviour if both sides in the migration have different
configuration files. So maybe we would need the configuration file
options listed in the criu log for RPC. Overwriting the RPC options with
the configuration file means that we are using it the other way round
then from CLI:

 CLI: first configuration file then CLI
 RPC: first RPC option then configuration file

Another unclear point is what to do with options that be specified
multiple times. Like verbosity=2 in the configuration file and two more
verbosity options on the command line. For the CLI this means -v4 in the
end right now. How to handle this case with RPC.

So, please let us know what you think and what would be the 'right'
solution.

		Adrian