Use caddy's certmagic library for extensible/robust ACME handling (#14177)

* use certmagic for more extensible/robust ACME cert handling

* accept TOS based on config option

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/golang.org/x/net/ipv6/header.go

@@ -0,0 +1,55 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ipv6
+
+import (
+	"encoding/binary"
+	"fmt"
+	"net"
+)
+
+const (
+	Version   = 6  // protocol version
+	HeaderLen = 40 // header length
+)
+
+// A Header represents an IPv6 base header.
+type Header struct {
+	Version      int    // protocol version
+	TrafficClass int    // traffic class
+	FlowLabel    int    // flow label
+	PayloadLen   int    // payload length
+	NextHeader   int    // next header
+	HopLimit     int    // hop limit
+	Src          net.IP // source address
+	Dst          net.IP // destination address
+}
+
+func (h *Header) String() string {
+	if h == nil {
+		return "<nil>"
+	}
+	return fmt.Sprintf("ver=%d tclass=%#x flowlbl=%#x payloadlen=%d nxthdr=%d hoplim=%d src=%v dst=%v", h.Version, h.TrafficClass, h.FlowLabel, h.PayloadLen, h.NextHeader, h.HopLimit, h.Src, h.Dst)
+}
+
+// ParseHeader parses b as an IPv6 base header.
+func ParseHeader(b []byte) (*Header, error) {
+	if len(b) < HeaderLen {
+		return nil, errHeaderTooShort
+	}
+	h := &Header{
+		Version:      int(b[0]) >> 4,
+		TrafficClass: int(b[0]&0x0f)<<4 | int(b[1])>>4,
+		FlowLabel:    int(b[1]&0x0f)<<16 | int(b[2])<<8 | int(b[3]),
+		PayloadLen:   int(binary.BigEndian.Uint16(b[4:6])),
+		NextHeader:   int(b[6]),
+		HopLimit:     int(b[7]),
+	}
+	h.Src = make(net.IP, net.IPv6len)
+	copy(h.Src, b[8:24])
+	h.Dst = make(net.IP, net.IPv6len)
+	copy(h.Dst, b[24:40])
+	return h, nil
+}