use a trampoline to get to the spicy _x functions

internal/msgx/doc.go

@@ -0,0 +1,5 @@
+// Package msgx calls xnu's private batched socket calls, sendmsg_x and recvmsg_x, through libSystem.
+//
+// The calls are not public API, so the package never links them: it looks them up with dlsym on first use.
+// The package is darwin-only, and on iOS Available is always false, so App Store builds keep to public API.
+package msgx