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.

ns/ns.go

@@ -0,0 +1,81 @@
+// 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 ns
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"syscall"
+)
+
+var setNsMap = map[string]uintptr{
+	"386":   346,
+	"amd64": 308,
+	"arm":   374,
+}
+
+// SetNS sets the network namespace on a target file.
+func SetNS(f *os.File, flags uintptr) error {
+	if runtime.GOOS != "linux" {
+		return fmt.Errorf("unsupported OS: %s", runtime.GOOS)
+	}
+
+	trap, ok := setNsMap[runtime.GOARCH]
+	if !ok {
+		return fmt.Errorf("unsupported arch: %s", runtime.GOARCH)
+	}
+
+	_, _, err := syscall.RawSyscall(trap, f.Fd(), flags, 0)
+	if err != 0 {
+		return err
+	}
+
+	return nil
+}
+
+// WithNetNSPath executes the passed closure under the given network
+// namespace, restoring the original namespace afterwards.
+func WithNetNSPath(nspath string, f func(*os.File) error) error {
+	ns, err := os.Open(nspath)
+	if err != nil {
+		return fmt.Errorf("Failed to open %v: %v", nspath, err)
+	}
+	defer ns.Close()
+
+	return WithNetNS(ns, f)
+}
+
+// WithNetNS executes the passed closure under the given network
+// namespace, restoring the original namespace afterwards.
+func WithNetNS(ns *os.File, f func(*os.File) error) error {
+	// save a handle to current (host) network namespace
+	thisNS, err := os.Open("/proc/self/ns/net")
+	if err != nil {
+		return fmt.Errorf("Failed to open /proc/self/ns/net: %v", err)
+	}
+	defer thisNS.Close()
+
+	if err = SetNS(ns, syscall.CLONE_NEWNET); err != nil {
+		return fmt.Errorf("Error switching to ns %v: %v", ns.Name(), err)
+	}
+
+	if err = f(thisNS); err != nil {
+		return err
+	}
+
+	// switch back
+	return SetNS(thisNS, syscall.CLONE_NEWNET)
+}