fix(os_fingerprint): set ip_no_pmtu_disc=1 for windows to eliminate TI=Z

When ip_no_pmtu_disc=0 the Linux kernel sets DF=1 on TCP packets and uses
IP ID=0 (RFC 6864). nmap's TI=Z fingerprint has no Windows match in its DB,
causing 91% confidence guesses of 'Linux 2.4/2.6 embedded' regardless of
TTL being 128. Setting ip_no_pmtu_disc=1 allows non-zero IP ID generation.

Trade-off: DF bit is not set on outgoing packets (slightly wrong for Windows)
but TI=Z is far more damaging to the spoof than losing DF accuracy.

decnet/os_fingerprint.py

@@ -48,7 +48,7 @@ OS_SYSCTLS: dict[str, dict[str, str]] = {
         "net.ipv4.tcp_window_scaling": "1",
         "net.ipv4.tcp_sack": "1",
         "net.ipv4.tcp_ecn": "0",
-        "net.ipv4.ip_no_pmtu_disc": "0",
+        "net.ipv4.ip_no_pmtu_disc": "1",   # avoid TI=Z: forces non-zero IP IDs
         "net.ipv4.tcp_fin_timeout": "30",
     },
     "bsd": {