libcontainer: Ensure bind mount target files are inside rootfs

Before we create any files to bind-mount on, make sure they are
inside the container rootfs, handling for instance absolute symbolic
links inside the container.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Move Follow symlink to pkg
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Update code to handle new path to Follow Symlink func
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Add MAINTAINERS file to symlink pkg
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #5778 from crosbymichael/check-symlink

Ensure libcontainer follows correct symlink in scope