Fix external link on security of containers

Docker-DCO-1.1-Signed-off-by: Vladimir Rutsky <altsysrq@gmail.com> (github: rutsky)
Merge pull request #4698 from rutsky/external_link_in_rst_fix

Fix external link on security of containers
Add reexec pkg to allow a binary to be reexecd

This allows one binary to be execed with different
argv[0]'s
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Remove the concept of a root dir out of engine

This makes the engine more general purpose so that we can
use it and the job routing functionality for reexec'ing our binary
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Add sysinit as a job
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Remove ReplaceEnv and use Env.Init instead
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)