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/tcp/tcp_windows.go

@@ -0,0 +1,37 @@
+package tcp
+
+import (
+	"syscall"
+	"time"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+type fileDescriptor syscall.Handle
+
+func sendTo(fd fileDescriptor, p []byte, flags int, to syscall.Sockaddr) (err error) {
+	return syscall.Sendto(syscall.Handle(fd), p, flags, to)
+}
+
+func setSocketTimeout(fd fileDescriptor, timeout time.Duration) (err error) {
+	timeval := int(timeout.Milliseconds())
+	return syscall.SetsockoptInt(syscall.Handle(fd), syscall.SOL_SOCKET, windows.SO_RCVTIMEO, timeval)
+}
+
+func recvFrom(fd fileDescriptor, p []byte, flags int) (n int, from syscall.Sockaddr, err error) {
+	return syscall.Recvfrom(syscall.Handle(fd), p, flags)
+}
+
+func setMTUDiscovery(fd syscall.Handle) error {
+	panic("not implemented")
+}
+
+func setNonBlock(fd syscall.Handle) error {
+	// Windows: Use ioctlsocket with FIONBIO
+	var arg uint32 = 1 // 1 to enable non-blocking mode
+	var bytesReturned uint32
+	const FIONBIO = 0x8004667e
+	return syscall.WSAIoctl(fd, FIONBIO, (*byte)(unsafe.Pointer(&arg)),
+		uint32(unsafe.Sizeof(arg)), nil, 0, &bytesReturned, nil, 0)
+}