syscall: add if_announce support for netbsd

Update #4866.

R=golang-dev, bradfitz, dave
CC=golang-dev
https://golang.org/cl/7382052
diff --git a/src/pkg/syscall/zerrors_netbsd_386.go b/src/pkg/syscall/zerrors_netbsd_386.go
index 3a5967c..68051f9 100644
--- a/src/pkg/syscall/zerrors_netbsd_386.go
+++ b/src/pkg/syscall/zerrors_netbsd_386.go
@@ -572,6 +572,8 @@
 	ICANON                            = 0x100
 	ICRNL                             = 0x100
 	IEXTEN                            = 0x400
+	IFAN_ARRIVAL                      = 0x0
+	IFAN_DEPARTURE                    = 0x1
 	IFA_ROUTE                         = 0x1
 	IFF_ALLMULTI                      = 0x200
 	IFF_BROADCAST                     = 0x2