Factor an API out into a module

This takes some of the machinery from CNI and from the rkt networking
code, and turns it into a library that can be linked into go apps.

Included is an example command-line application that uses the library,
called `cnitool`.

Other headline changes:

 * Plugin exec'ing is factored out

The motivation here is to factor out the protocol for invoking
plugins. To that end, a generalisation of the code from api.go and
pkg/plugin/ipam.go goes into pkg/invoke/exec.go.

 * Move argument-handling and conf-loading into public API

The fact that the arguments get turned into an environment for the
plugin is incidental to the API; so, provide a way of supplying them
as a struct or saying "just use the same arguments as I got" (the
latter is for IPAM plugins).

cnitool/cni.go

@@ -0,0 +1,87 @@
+// Copyright 2015 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 main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/appc/cni/libcni"
+)
+
+const (
+	EnvCNIPath = "CNI_PATH"
+	EnvNetDir  = "NETCONFPATH"
+
+	DefaultNetDir = "/etc/cni/net.d"
+
+	CmdAdd = "add"
+	CmdDel = "del"
+)
+
+func main() {
+	if len(os.Args) < 3 {
+		usage()
+		return
+	}
+
+	netdir := os.Getenv(EnvNetDir)
+	if netdir == "" {
+		netdir = DefaultNetDir
+	}
+	netconf, err := libcni.LoadConf(netdir, os.Args[2])
+	if err != nil {
+		exit(err)
+	}
+
+	netns := os.Args[3]
+
+	cninet := &libcni.CNIConfig{
+		Path: strings.Split(os.Getenv(EnvCNIPath), ":"),
+	}
+
+	rt := &libcni.RuntimeConf{
+		ContainerID: "cni",
+		NetNS:       netns,
+		IfName:      "eth0",
+	}
+
+	switch os.Args[1] {
+	case CmdAdd:
+		_, err := cninet.AddNetwork(netconf, rt)
+		exit(err)
+	case CmdDel:
+		exit(cninet.DelNetwork(netconf, rt))
+	}
+}
+
+func usage() {
+	exe := filepath.Base(os.Args[0])
+
+	fmt.Fprintf(os.Stderr, "%s: Add or remove network interfaces from a network namespace\n", exe)
+	fmt.Fprintf(os.Stderr, "  %s %s <net> <netns>\n", exe, CmdAdd)
+	fmt.Fprintf(os.Stderr, "  %s %s <net> <netns>\n", exe, CmdDel)
+	os.Exit(1)
+}
+
+func exit(err error) {
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%s\n", err)
+		os.Exit(1)
+	}
+	os.Exit(0)
+}