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/README.md

@@ -0,0 +1,51 @@
+# netns - network namespaces in go #
+
+The netns package provides an ultra-simple interface for handling
+network namespaces in go. Changing namespaces requires elevated
+privileges, so in most cases this code needs to be run as root.
+
+## Local Build and Test ##
+
+You can use go get command:
+
+    go get github.com/vishvananda/netns
+
+Testing (requires root):
+
+    sudo -E go test github.com/vishvananda/netns
+
+## Example ##
+
+```go
+package main
+
+import (
+    "fmt"
+    "net"
+    "runtime"
+    "github.com/vishvananda/netns"
+)
+
+func main() {
+    // Lock the OS Thread so we don't accidentally switch namespaces
+    runtime.LockOSThread()
+    defer runtime.UnlockOSThread()
+
+    // Save the current network namespace
+    origns, _ := netns.Get()
+    defer origns.Close()
+
+    // Create a new network namespace
+    newns, _ := netns.New()
+    netns.Set(newns)
+    defer newns.Close()
+
+    // Do something with the network namespace
+    ifaces, _ := net.Interfaces()
+    fmt.Printf("Interfaces: %v\n", ifaces)
+
+    // Switch back to the original namespace
+    netns.Set(origns)
+}
+
+```