use a trampoline to get to the spicy _x functions

internal/msgx/hdr_darwin.go

@@ -0,0 +1,16 @@
+package msgx
+
+import "golang.org/x/sys/unix"
+
+// Hdr mirrors xnu's struct msghdr_x (bsd/sys/socket_private.h), which the SDK doesn't ship.
+// recvmsg_x reports each message's length in Datalen rather than in its return value.
+type Hdr struct {
+	Name       *byte
+	Namelen    uint32
+	Iov        *unix.Iovec
+	Iovlen     int32
+	Control    *byte
+	Controllen uint32
+	Flags      int32
+	Datalen    uint64
+}