Path MTU discovery fixes and improvements (#3109)

- Existing option `WIREGUARD_MTU` , if set, disables PMTUD and is used
- New option `PMTUD_ICMP_ADDRESSES=1.1.1.1,8.8.8.8` and `PMTUD_TCP_ADDRESSES=1.1.1.1:443,8.8.8.8:443`
- ICMP PMTUD now targets external-by-default IP addresses
- New TCP PMTUD (binary search only) as a second MTU confirmation and fallback mechanism.
- Force set TCP MSS to MTU - IP header - TCP base header - "magic 20 bytes" 🎆
- Fix #3108

.golangci.yml

@@ -22,6 +22,7 @@ linters:
         - "^disabled$"
         # Firewall and routing strings
         - "^(ACCEPT|DROP)$"
+        - "^--append$"
         - "^--delete$"
         - "^all$"
         - "^(tcp|udp)$"