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).

libcni/api.go

@@ -0,0 +1,68 @@
+// 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 libcni
+
+import (
+	"strings"
+
+	"github.com/appc/cni/pkg/invoke"
+	"github.com/appc/cni/pkg/types"
+)
+
+type RuntimeConf struct {
+	ContainerID string
+	NetNS       string
+	IfName      string
+	Args        [][2]string
+}
+
+type NetworkConfig struct {
+	Network *types.NetConf
+	Bytes   []byte
+}
+
+type CNI interface {
+	AddNetwork(net *NetworkConfig, rt *RuntimeConf) (*types.Result, error)
+	DelNetwork(net *NetworkConfig, rt *RuntimeConf) error
+}
+
+type CNIConfig struct {
+	Path []string
+}
+
+func (c *CNIConfig) AddNetwork(net *NetworkConfig, rt *RuntimeConf) (*types.Result, error) {
+	return c.execPlugin("ADD", net, rt)
+}
+
+func (c *CNIConfig) DelNetwork(net *NetworkConfig, rt *RuntimeConf) error {
+	_, err := c.execPlugin("DEL", net, rt)
+	return err
+}
+
+// =====
+
+func (c *CNIConfig) execPlugin(action string, conf *NetworkConfig, rt *RuntimeConf) (*types.Result, error) {
+	pluginPath := invoke.FindInPath(conf.Network.Type, c.Path)
+
+	args := &invoke.Args{
+		Command:     action,
+		ContainerID: rt.ContainerID,
+		NetNS:       rt.NetNS,
+		PluginArgs:  rt.Args,
+		IfName:      rt.IfName,
+		Path:        strings.Join(c.Path, ":"),
+	}
+	return invoke.ExecPlugin(pluginPath, conf.Bytes, args)
+}