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/ipv6_linux.go

@@ -0,0 +1,8 @@
+package ip
+
+import "syscall"
+
+func SetIPv6HeaderIncluded(fd int) error {
+	const ipv6HdrIncluded = 36 // IPV6_HDRINCL
+	return syscall.SetsockoptInt(fd, syscall.IPPROTO_IPV6, ipv6HdrIncluded, 1)
+}