Do it through new --weak-sysctls option to "criu restore".

When we migrate from source with newer kernel with some modern sysctl
to destination with older kernel without it or just with a kernel
configured without it (e.g. no CONFIG_IPV6_ROUTER_PREF), we face a
problem that not all of dumped sysctls can be successfuly restored
on the destination. So to address these problem introduce weak sysctls
migration, where we restore only available sysctls and skip others.

https://github.com/xemul/criu/issues/248

While on it, fix workaround for stable_secret eio skip.

travis-ci: success for Add ability to restore with missing sysctls on the target node
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>