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.go

@@ -0,0 +1,10 @@
+//go:build !linux && !windows
+
+package icmp
+
+// setDontFragment for platforms other than Linux and Windows
+// is not implemented, so we just return assuming the don't
+// fragment flag is set on IP packets.
+func setDontFragment(fd uintptr) (err error) {
+	return nil
+}