hotfix(pmtud/tcp): block kernel from racing to send RST packets

- this makes PMTUD TCP reliable
- this only works on kernels with the mark module
- on kernels without the mark module, the icmp pmtud mtu found is used

internal/pmtud/tcp/tcp_unspecified.go

@@ -2,6 +2,10 @@
 
 package tcp
 
+func setMark(fd, excludeMark int) error {
+	panic("not implemented")
+}
+
 func setMTUDiscovery(fd int) error {
 	panic("not implemented")
 }