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.

plugins/ipam/host-local/backend/disk/backend.go

@@ -0,0 +1,88 @@
+// 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 disk
+
+import (
+	"io/ioutil"
+	"net"
+	"os"
+	"path/filepath"
+)
+
+var defaultDataDir = "/var/lib/cni/networks"
+
+type Store struct {
+	FileLock
+	dataDir string
+}
+
+func New(network string) (*Store, error) {
+	dir := filepath.Join(defaultDataDir, network)
+	if err := os.MkdirAll(dir, 0644); err != nil {
+		return nil, err
+	}
+
+	lk, err := NewFileLock(dir)
+	if err != nil {
+		return nil, err
+	}
+	return &Store{*lk, dir}, nil
+}
+
+func (s *Store) Reserve(id string, ip net.IP) (bool, error) {
+	fname := filepath.Join(s.dataDir, ip.String())
+	f, err := os.OpenFile(fname, os.O_RDWR|os.O_EXCL|os.O_CREATE, 0644)
+	if os.IsExist(err) {
+		return false, nil
+	}
+	if err != nil {
+		return false, err
+	}
+	if _, err := f.WriteString(id); err != nil {
+		f.Close()
+		os.Remove(f.Name())
+		return false, err
+	}
+	if err := f.Close(); err != nil {
+		os.Remove(f.Name())
+		return false, err
+	}
+	return true, nil
+}
+
+func (s *Store) Release(ip net.IP) error {
+	return os.Remove(filepath.Join(s.dataDir, ip.String()))
+}
+
+// N.B. This function eats errors to be tolerant and
+// release as much as possible
+func (s *Store) ReleaseByID(id string) error {
+	err := filepath.Walk(s.dataDir, func(path string, info os.FileInfo, err error) error {
+		if err != nil || info.IsDir() {
+			return nil
+		}
+		data, err := ioutil.ReadFile(path)
+		if err != nil {
+			return nil
+		}
+		if string(data) == id {
+			if err := os.Remove(path); err != nil {
+				return nil
+			}
+		}
+		return nil
+	})
+	return err
+}