net: fix cgoAddrInfoFlags() on FreeBSD

        CL 6250075 removed AI_MASK mask on all BSD variants,
        however FreeBSD's AI_MASK does not include AI_V4MAPPED
        and AI_ALL, and its libc is strict about the ai_flags.

        This will fix the FreeBSD builder.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6305054

src/pkg/net/cgo_bsd.go

@@ -12,5 +12,5 @@ package net
 import "C"
 
 func cgoAddrInfoFlags() C.int {
-	return C.AI_CANONNAME | C.AI_V4MAPPED | C.AI_ALL
+	return (C.AI_CANONNAME | C.AI_V4MAPPED | C.AI_ALL) & C.AI_MASK
 }