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

internal/pmtud/ip/ipv4_windows.go

@@ -0,0 +1,12 @@
+package ip
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+func SetIPv4HeaderIncluded(handle syscall.Handle) error {
+	const ipHdrIncluded = windows.IP_HDRINCL
+	return syscall.SetsockoptInt(handle, syscall.IPPROTO_IP, ipHdrIncluded, 1)
+}