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/icmp/df_windows.go

@@ -0,0 +1,11 @@
+package icmp
+
+import (
+	"syscall"
+)
+
+func setDontFragment(fd uintptr) (err error) {
+	// https://docs.microsoft.com/en-us/troubleshoot/windows/win32/header-library-requirement-socket-ipproto-ip
+	// #define IP_DONTFRAGMENT        14     /* don't fragment IP datagrams */
+	return syscall.SetsockoptInt(syscall.Handle(fd), syscall.IPPROTO_IP, 14, 1)
+}