Adds support for Setuid/Setgid calls that has been removed from go 1.4

This will be required to make libcontainer work with go 1.4.
I tested that changing these calls in namespaces/init.go works with this commit against go HEAD.

Docker-DCO-1.1-Signed-off-by: Mrunal Patel <mrunalp@gmail.com> (github: mrunalp)
arguably this could be its own file no? that way we don't duplicate it