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/http2/headermap.go

@@ -0,0 +1,105 @@
+// 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 http2
+
+import (
+	"net/http"
+	"sync"
+)
+
+var (
+	commonBuildOnce   sync.Once
+	commonLowerHeader map[string]string // Go-Canonical-Case -> lower-case
+	commonCanonHeader map[string]string // lower-case -> Go-Canonical-Case
+)
+
+func buildCommonHeaderMapsOnce() {
+	commonBuildOnce.Do(buildCommonHeaderMaps)
+}
+
+func buildCommonHeaderMaps() {
+	common := []string{
+		"accept",
+		"accept-charset",
+		"accept-encoding",
+		"accept-language",
+		"accept-ranges",
+		"age",
+		"access-control-allow-credentials",
+		"access-control-allow-headers",
+		"access-control-allow-methods",
+		"access-control-allow-origin",
+		"access-control-expose-headers",
+		"access-control-max-age",
+		"access-control-request-headers",
+		"access-control-request-method",
+		"allow",
+		"authorization",
+		"cache-control",
+		"content-disposition",
+		"content-encoding",
+		"content-language",
+		"content-length",
+		"content-location",
+		"content-range",
+		"content-type",
+		"cookie",
+		"date",
+		"etag",
+		"expect",
+		"expires",
+		"from",
+		"host",
+		"if-match",
+		"if-modified-since",
+		"if-none-match",
+		"if-unmodified-since",
+		"last-modified",
+		"link",
+		"location",
+		"max-forwards",
+		"origin",
+		"proxy-authenticate",
+		"proxy-authorization",
+		"range",
+		"referer",
+		"refresh",
+		"retry-after",
+		"server",
+		"set-cookie",
+		"strict-transport-security",
+		"trailer",
+		"transfer-encoding",
+		"user-agent",
+		"vary",
+		"via",
+		"www-authenticate",
+		"x-forwarded-for",
+		"x-forwarded-proto",
+	}
+	commonLowerHeader = make(map[string]string, len(common))
+	commonCanonHeader = make(map[string]string, len(common))
+	for _, v := range common {
+		chk := http.CanonicalHeaderKey(v)
+		commonLowerHeader[chk] = v
+		commonCanonHeader[v] = chk
+	}
+}
+
+func lowerHeader(v string) (lower string, ascii bool) {
+	buildCommonHeaderMapsOnce()
+	if s, ok := commonLowerHeader[v]; ok {
+		return s, true
+	}
+	return asciiToLower(v)
+}
+
+func canonicalHeader(v string) string {
+	buildCommonHeaderMapsOnce()
+	if s, ok := commonCanonHeader[v]; ok {
+		return s
+	}
+	return http.CanonicalHeaderKey(v)
+}