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_notdarwin.go

@@ -0,0 +1,30 @@
+//go:build !darwin
+
+package tcp
+
+import (
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+)
+
+func stripIPv4Header(reply []byte) (result []byte, ok bool) {
+	if len(reply) < int(constants.IPv4HeaderLength) {
+		return nil, false // not an IPv4 packet
+	}
+
+	version := reply[0] >> 4 //nolint:mnd
+	const ipv4Version = 4
+	if version != ipv4Version {
+		return nil, false
+	}
+	// For IPv4 we need to skip the IP header, which is at least
+	// 20B and can be up to 60B.
+	// The Internet Header Length is the lower 4 bits of the first byte and
+	// represents the number of 32-bit words of the header length.
+	const ihlMask byte = 0x0F
+	const bytesInWord = 4
+	headerLength := int((reply[0] & ihlMask)) * bytesInWord
+	if len(reply) < headerLength {
+		return nil, false // not enough data for full IPv4 header
+	}
+	return reply[headerLength:], true
+}