Move shim client into subpackage

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

linux/shim/client/client_unix.go

@@ -0,0 +1,18 @@
+// +build !linux,!windows
+
+package client
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+func getSysProcAttr(nonewns bool) *syscall.SysProcAttr {
+	return &syscall.SysProcAttr{
+		Setpgid: true,
+	}
+}
+
+func setCgroup(cgroupPath string, cmd *exec.Cmd) error {
+	return nil
+}