Add support for negotiating IMAP and SMTP on the HTTPS port 443 using TLS ALPN "imap" and "smtp"

Intended for future use with chatmail servers. Standard email ports may be
blocked on some networks, while the HTTPS port may be accessible.

This is a squashed commit of PR #255 by s0ph0s-dog.

vendor/golang.org/x/net/http/httpguts/guts.go

@@ -0,0 +1,50 @@
+// Copyright 2018 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 httpguts provides functions implementing various details
+// of the HTTP specification.
+//
+// This package is shared by the standard library (which vendors it)
+// and x/net/http2. It comes with no API stability promise.
+package httpguts
+
+import (
+	"net/textproto"
+	"strings"
+)
+
+// ValidTrailerHeader reports whether name is a valid header field name to appear
+// in trailers.
+// See RFC 7230, Section 4.1.2
+func ValidTrailerHeader(name string) bool {
+	name = textproto.CanonicalMIMEHeaderKey(name)
+	if strings.HasPrefix(name, "If-") || badTrailer[name] {
+		return false
+	}
+	return true
+}
+
+var badTrailer = map[string]bool{
+	"Authorization":       true,
+	"Cache-Control":       true,
+	"Connection":          true,
+	"Content-Encoding":    true,
+	"Content-Length":      true,
+	"Content-Range":       true,
+	"Content-Type":        true,
+	"Expect":              true,
+	"Host":                true,
+	"Keep-Alive":          true,
+	"Max-Forwards":        true,
+	"Pragma":              true,
+	"Proxy-Authenticate":  true,
+	"Proxy-Authorization": true,
+	"Proxy-Connection":    true,
+	"Range":               true,
+	"Realm":               true,
+	"Te":                  true,
+	"Trailer":             true,
+	"Transfer-Encoding":   true,
+	"Www-Authenticate":    true,
+}