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).

types/args.go

@@ -0,0 +1,50 @@
+// 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 types
+
+import (
+	"encoding"
+	"fmt"
+	"reflect"
+	"strings"
+)
+
+func LoadArgs(args string, container interface{}) error {
+	if args == "" {
+		return nil
+	}
+
+	containerValue := reflect.ValueOf(container)
+
+	pairs := strings.Split(args, ";")
+	for _, pair := range pairs {
+		kv := strings.Split(pair, "=")
+		if len(kv) != 2 {
+			return fmt.Errorf("ARGS: invalid pair %q", pair)
+		}
+		keyString := kv[0]
+		valueString := kv[1]
+		keyField := containerValue.Elem().FieldByName(keyString)
+		if !keyField.IsValid() {
+			return fmt.Errorf("ARGS: invalid key %q", keyString)
+		}
+		u := keyField.Addr().Interface().(encoding.TextUnmarshaler)
+		err := u.UnmarshalText([]byte(valueString))
+		if err != nil {
+			return fmt.Errorf("ARGS: error parsing value of pair %q: %v)", pair, err)
+		}
+	}
+	return nil
+}