[CRIU] Additional patches on top of 'add support for external net namespaces'

[Adrian Reber]

This adds support to use 'external net namespaces'via RPC by
correctly evaluating the inherit-fd RPC option.

It also introduces a fake feature check which is used in the runc test
suite to make sure the CRIU in the test suite support 'external net
namespaces' before running the test.

The last patch is a simple test change to support running the
test also on CentOS.

[PATCH 1/3] cr-service: open FDs passed via inherit_fd option also in
[PATCH 2/3] check: add a feature check for external network
[PATCH 3/3] netns_ext: make test work on CentOS