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/lock.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 disk
+
+import (
+	"os"
+	"syscall"
+)
+
+// FileLock wraps os.File to be used as a lock using flock
+type FileLock struct {
+	f *os.File
+}
+
+// NewFileLock opens file/dir at path and returns unlocked FileLock object
+func NewFileLock(path string) (*FileLock, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+
+	return &FileLock{f}, nil
+}
+
+// Close closes underlying file
+func (l *FileLock) Close() error {
+	return l.f.Close()
+}
+
+// Lock acquires an exclusive lock
+func (l *FileLock) Lock() error {
+	return syscall.Flock(int(l.f.Fd()), syscall.LOCK_EX)
+}
+
+// Unlock releases the lock
+func (l *FileLock) Unlock() error {
+	return syscall.Flock(int(l.f.Fd()), syscall.LOCK_UN)
+}