check: add a feature check for external network namespaces

This is obviously not a real check. This only exists, so that CRIU
clients/users can check if this CRIU version supports the external
network namespace feature. Theoretically the CRIU client or user could
also parse the version, but especially for CLI users version comparison
in the shell is not easy.

This feature check does not exist for RPC as RPC has a special version
call which does not require string parsing and the external network
namespace feature is available for all CRIU versions newer than 3.9.

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>