Add plugin code

This adds basic plugins.
"main" types: veth, bridge, macvlan
"ipam" type: host-local

The code has been ported over from github.com/coreos/rkt project
and adapted to fit the CNI spec.

skel/skel.go

@@ -0,0 +1,98 @@
+// Copyright 2014 CoreOS, Inc.
+//
+// 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 skel provides skeleton code for a CNI plugin.
+// In particular, it implements argument parsing and validation.
+package skel
+
+import (
+	"io/ioutil"
+	"log"
+	"os"
+)
+
+// CmdArgs captures all the arguments passed in to the plugin
+// via both env vars and stdin
+type CmdArgs struct {
+	ContainerID string
+	Netns       string
+	IfName      string
+	Args        string
+	Path        string
+	StdinData   []byte
+}
+
+// PluginMain is the "main" for a plugin. It accepts
+// two callback functions for add and del commands.
+func PluginMain(cmdAdd, cmdDel func(_ *CmdArgs) error) {
+	var cmd, contID, netns, ifName, args, path string
+
+	vars := []struct {
+		name string
+		val  *string
+		req  bool
+	}{
+		{"CNI_COMMAND", &cmd, true},
+		{"CNI_CONTAINERID", &contID, false},
+		{"CNI_NETNS", &netns, true},
+		{"CNI_IFNAME", &ifName, true},
+		{"CNI_ARGS", &args, false},
+		{"CNI_PATH", &path, true},
+	}
+
+	argsMissing := false
+	for _, v := range vars {
+		*v.val = os.Getenv(v.name)
+		if v.req && *v.val == "" {
+			log.Printf("%v env variable missing", v.name)
+			argsMissing = true
+		}
+	}
+
+	if argsMissing {
+		os.Exit(1)
+	}
+
+	stdinData, err := ioutil.ReadAll(os.Stdin)
+	if err != nil {
+		log.Printf("Error reading from stdin: %v", err)
+		os.Exit(1)
+	}
+
+	cmdArgs := &CmdArgs{
+		ContainerID: contID,
+		Netns:       netns,
+		IfName:      ifName,
+		Args:        args,
+		Path:        path,
+		StdinData:   stdinData,
+	}
+
+	switch cmd {
+	case "ADD":
+		err = cmdAdd(cmdArgs)
+
+	case "DEL":
+		err = cmdDel(cmdArgs)
+
+	default:
+		log.Printf("Unknown CNI_COMMAND: %v", cmd)
+		os.Exit(1)
+	}
+
+	if err != nil {
+		log.Printf("%v: %v", cmd, err)
+		os.Exit(1)
+	}
+}