libcontainer: Support mounting tmpfs

By adding a mount type of "tmpfs" you can tell libcontainer to mount a
fresh tmpfs instance at some point in the container. If CopyContent is
also set the existing files at the mountpoint will be copied into the
tmpfs.

This will be used from docker to implement /run.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
So I guess the question is why is this only for tmpfs? Only concern I have is from an API perspective where some of these things start to become arbitrary. I am fine saying that we only do it for tmpfs for the moment.