Add error message for ifname collision (#74)

* Add error message for ifname collision

This changes introduce to validateIfName, to check the validation
for interface name, i.e. same name among two or more interfaces.

vendor/github.com/vishvananda/netns/netns_unspecified.go

@@ -0,0 +1,43 @@
+// +build !linux
+
+package netns
+
+import (
+	"errors"
+)
+
+var (
+	ErrNotImplemented = errors.New("not implemented")
+)
+
+func Set(ns NsHandle) (err error) {
+	return ErrNotImplemented
+}
+
+func New() (ns NsHandle, err error) {
+	return -1, ErrNotImplemented
+}
+
+func Get() (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromPath(path string) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromName(name string) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromPid(pid int) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromThread(pid, tid int) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromDocker(id string) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}