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).

pkg/invoke/find.go

@@ -0,0 +1,37 @@
+// 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 invoke
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func FindInPath(plugin string, path []string) string {
+	for _, p := range path {
+		fullname := filepath.Join(p, plugin)
+		if fi, err := os.Stat(fullname); err == nil && fi.Mode().IsRegular() {
+			return fullname
+		}
+	}
+	return ""
+}
+
+// Find returns the full path of the plugin by searching in CNI_PATH
+func Find(plugin string) string {
+	paths := strings.Split(os.Getenv("CNI_PATH"), ":")
+	return FindInPath(plugin, paths)
+}