feat(vpn): path MTU discovery to find the best MTU (#2586)

internal/pmtud/df.go

@@ -0,0 +1,10 @@
+//go:build !linux && !windows
+
+package pmtud
+
+// setDontFragment for platforms other than Linux and Windows
+// is not implemented, so we just return assuming the don't
+// fragment flag is set on IP packets.
+func setDontFragment(fd uintptr) (err error) {
+	return nil
+}