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)
Should we say that this is only for tmpfs?