Improve general quality of libcontainer

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)

system/setns_linux.go

@@ -0,0 +1,13 @@
+package system
+
+import (
+	"syscall"
+)
+
+func Setns(fd uintptr, flags uintptr) error {
+	_, _, err := syscall.RawSyscall(SYS_SETNS, fd, flags, 0)
+	if err != 0 {
+		return err
+	}
+	return nil
+}