syscall: add sockaddr_dl, sysctl with routing message support for darwin, freebsd

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4171043
diff --git a/src/pkg/syscall/syscall_freebsd.go b/src/pkg/syscall/syscall_freebsd.go
index ee947be..ed31066 100644
--- a/src/pkg/syscall/syscall_freebsd.go
+++ b/src/pkg/syscall/syscall_freebsd.go
@@ -14,6 +14,18 @@
 
 const OS = "freebsd"
 
+type SockaddrDatalink struct {
+	Len    uint8
+	Family uint8
+	Index  uint16
+	Type   uint8
+	Nlen   uint8
+	Alen   uint8
+	Slen   uint8
+	Data   [46]int8
+	raw    RawSockaddrDatalink
+}
+
 /*
  * Exposed directly
  */