Adds forkexec library that supports user namespaces in Go.

This PR adds 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)
Should we drop this code? Since we don't care about OSes that don't support user namespaces?