use system.Set{u,g}id to fix Set{u,g}id on Go 1.4

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #251 from unclejack/fix_go1.4_syscalls

use system.Set{u,g}id to fix Set{u,g}id on Go 1.4