pkg/ns: refactored so that builds succeed on non-linux platforms

moved functions that depend on linux packages (sys/unix) into a separate file
and added nop methods with a build tag for non-linux platforms in a new file.

ns/ns_unspecified.go

@@ -0,0 +1,33 @@
+// Copyright 2015-2017 CNI authors
+//
+// 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.
+
+// +build !linux
+
+package ns
+
+func getCurrentThreadNetNSPath() string {
+	return ""
+}
+
+func NewNS() (NetNS, error) {
+	return nil, NotImplementedError
+}
+
+func (ns *netNS) Close() error {
+	return NotImplementedError
+}
+
+func (ns *netNS) Set() error {
+	return NotImplementedError
+}