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.
Fixes incorrect API spec for items moved to HostConfig

DNS and VolumesFrom were moved to HostConfig and as such are part of the
container start and not create.
For some reason 0.10 docs are correct (except for a missing quote in the
JSON") but 0.11 and latest are not.

Docker-DCO-1.1-Signed-off-by: cpuguy83 <cpuguy83@gmail.com> (github: cpuguy83)
Merge pull request #6215 from cpuguy83/fix_volumes_from_api_doc

Fixes incorrect API spec for items moved to HostConfig