Enable Windows CI (Appveyor)

- start list of linux_only plugins; ignore them when testing on Windows
- Isolate linux-only code by filename suffix
- Remove stub (NotImplemented) functions
- other misc. fixes for Windows compatibility

pkg/ip/ipforward_linux.go

@@ -0,0 +1,55 @@
+// Copyright 2015 CNI authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ip
+
+import (
+	"io/ioutil"
+
+	"github.com/containernetworking/cni/pkg/types/current"
+)
+
+func EnableIP4Forward() error {
+	return echo1("/proc/sys/net/ipv4/ip_forward")
+}
+
+func EnableIP6Forward() error {
+	return echo1("/proc/sys/net/ipv6/conf/all/forwarding")
+}
+
+// EnableForward will enable forwarding for all configured
+// address families
+func EnableForward(ips []*current.IPConfig) error {
+	v4 := false
+	v6 := false
+
+	for _, ip := range ips {
+		if ip.Version == "4" && !v4 {
+			if err := EnableIP4Forward(); err != nil {
+				return err
+			}
+			v4 = true
+		} else if ip.Version == "6" && !v6 {
+			if err := EnableIP6Forward(); err != nil {
+				return err
+			}
+			v6 = true
+		}
+	}
+	return nil
+}
+
+func echo1(f string) error {
+	return ioutil.WriteFile(f, []byte("1"), 0644)
+}