Adding initial version of C-based nsenter for allowing execin in
libcontainer.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Move env load to nsenter
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Exclude the user namespace for setns
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #1 from crosbymichael/nsenter-path

Move environment loading to nsenter
Move doc section in Remote API about removing insert from v1.11 to v1.12

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
Merge pull request #6210 from tiborvass/fix_doc_remove_insert_remote_api

Move doc section in Remote API about removing insert from v1.11 to v1.12
Merge pull request #6026 from vmarmol/nsenter

Adding initial version of C-based nsenter for allowing execin in libcontainer.