Add forklib that supports user namespaces in Go.

This PR created a library that could be used to clone into user namespaces and write uid/gid mappings.
(This is based on joint effort between me and crosbymichael).

Docker-DCO-1.1-Signed-off-by: Mrunal Patel <mrunalp@gmail.com> (github: mrunalp)
Fixed to match what is done in exec_unix.go