syscall: make LoadConnectEx not IPv4-specific on Windows

Calls to Socket in LoadConnectEx always fail with the error
"An address incompatible with the requested protocol was used"
when IPv4 is disabled on Windows. We can work around this by
detecting that specific error and retrying with an IPv6 socket.

Fixes #29759

Author: linsite

src/syscall/syscall_windows.go

@@ -14,6 +14,7 @@ import (
 	"internal/oserror"
 	"internal/race"
 	"internal/strconv"
+	strings "internal/stringslite"
 	"sync"
 	"unsafe"
 )
@@ -1097,10 +1098,16 @@ var connectExFunc struct {
 
 func LoadConnectEx() error {
 	connectExFunc.once.Do(func() {
-		var s Handle
-		s, connectExFunc.err = Socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)
-		if connectExFunc.err != nil {
-			return
+		s, err := Socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)
+		if err != nil {
+			// This can occur when IPv4 is disabled. See https://go.dev/issue/29759.
+			if strings.Index(err.Error(), "address incompatible with the requested protocol was used") >= 0 {
+				s, err = Socket(AF_INET6, SOCK_STREAM, IPPROTO_TCP)
+			}
+			connectExFunc.err = err
+			if err != nil {
+				return
+			}
 		}
 		defer CloseHandle(s)
 		var n uint32