net: move cgo address info flags to per-platform files

Move address info flags to per-platform files. This is needed to
enable cgo on NetBSD (and later OpenBSD), as some of the currently
used AI_* defines do not exist on these platforms.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6250075

src/pkg/net/cgo_bsd.go

@@ -11,6 +11,6 @@ package net
 */
 import "C"
 
-func cgoAddrInfoMask() C.int {
-	return C.AI_MASK
+func cgoAddrInfoFlags() C.int {
+	return C.AI_CANONNAME | C.AI_V4MAPPED | C.AI_ALL
 }

src/pkg/net/cgo_linux.go

@@ -9,6 +9,12 @@ package net
 */
 import "C"
 
-func cgoAddrInfoMask() C.int {
+func cgoAddrInfoFlags() C.int {
+	// NOTE(rsc): In theory there are approximately balanced
+	// arguments for and against including AI_ADDRCONFIG
+	// in the flags (it includes IPv4 results only on IPv4 systems,
+	// and similarly for IPv6), but in practice setting it causes
+	// getaddrinfo to return the wrong canonical name on Linux.
+	// So definitely leave it out.
 	return C.AI_CANONNAME | C.AI_V4MAPPED | C.AI_ALL
 }

src/pkg/net/cgo_unix.go

@@ -81,13 +81,7 @@ func cgoLookupIPCNAME(name string) (addrs []IP, cname string, err error, complet
 	var res *C.struct_addrinfo
 	var hints C.struct_addrinfo
 
-	// NOTE(rsc): In theory there are approximately balanced
-	// arguments for and against including AI_ADDRCONFIG
-	// in the flags (it includes IPv4 results only on IPv4 systems,
-	// and similarly for IPv6), but in practice setting it causes
-	// getaddrinfo to return the wrong canonical name on Linux.
-	// So definitely leave it out.
-	hints.ai_flags = (C.AI_ALL | C.AI_V4MAPPED | C.AI_CANONNAME) & cgoAddrInfoMask()
+	hints.ai_flags = cgoAddrInfoFlags()
 
 	h := C.CString(name)
 	defer C.free(unsafe.Pointer(h))