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/ipheader_darwin.go

@@ -0,0 +1,9 @@
+package ip
+
+import (
+	"encoding/binary"
+)
+
+func putUint16(b []byte, v uint16) {
+	binary.NativeEndian.PutUint16(b, v)
+}