Use new libcontainer.State API

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Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
We already have the new libcontainer API in vendor.sh, so neither "pid" nor "start" files would get created. Thus d.readStartTime(p) would always return a NotExist error. Now if you expand the lines below this comment you'll see that we always silence this error (the reason is in the code comment).