[CRIU] v2: automatic detection of external bind mounts

[Pavel Emelyanov]

> Great, I resent v4 of the patch without the check in it. I think I've
> covered all the review comments now, but please let me know if
> I've missed something.

Yup, at this point everything seems to be resolved, so I've applied the
set. Thank you :)

The next steps that I'd really appreciate would be tests for the new 
options we have and update of the RPC to support the slavery/sharing
enabling and auto mount-map mode. The latter is quite important as
Docker guys will use rpc-swrk mode for their dump-restore and will
definitely want to use this functionality.

-- Pavel