Use libcontainer cap drop method

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)

pkg/system/unsupported.go

@@ -28,3 +28,11 @@ func GetClockTicks() int {
 func CreateMasterAndConsole() (*os.File, string, error) {
 	return nil, "", ErrNotSupportedPlatform
 }
+
+func SetKeepCaps() error {
+	return ErrNotSupportedPlatform
+}
+
+func ClearKeepCaps() error {
+	return ErrNotSupportedPlatform
+}