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/helper.go

@@ -0,0 +1,58 @@
+// Copyright 2013 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 (
+	"errors"
+	"net"
+	"runtime"
+)
+
+var (
+	errInvalidConn     = errors.New("invalid connection")
+	errMissingAddress  = errors.New("missing address")
+	errHeaderTooShort  = errors.New("header too short")
+	errInvalidConnType = errors.New("invalid conn type")
+	errNotImplemented  = errors.New("not implemented on " + runtime.GOOS + "/" + runtime.GOARCH)
+)
+
+func boolint(b bool) int {
+	if b {
+		return 1
+	}
+	return 0
+}
+
+func netAddrToIP16(a net.Addr) net.IP {
+	switch v := a.(type) {
+	case *net.UDPAddr:
+		if ip := v.IP.To16(); ip != nil && ip.To4() == nil {
+			return ip
+		}
+	case *net.IPAddr:
+		if ip := v.IP.To16(); ip != nil && ip.To4() == nil {
+			return ip
+		}
+	}
+	return nil
+}
+
+func opAddr(a net.Addr) net.Addr {
+	switch a.(type) {
+	case *net.TCPAddr:
+		if a == nil {
+			return nil
+		}
+	case *net.UDPAddr:
+		if a == nil {
+			return nil
+		}
+	case *net.IPAddr:
+		if a == nil {
+			return nil
+		}
+	}
+	return a
+}