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.

testdata/integration/moxacmepebblealpn.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+set -x # print commands
+set -e # exit on failed command
+
+apk add curl
+
+(rm -r /tmp/mox 2>/dev/null || exit 0) # clean slate
+mkdir /tmp/mox
+cd /tmp/mox
+mox quickstart -skipdial moxtest1@mox1.example "$MOX_UID" > output.txt
+
+cp config/mox.conf config/mox.conf.orig
+sed -i -e 's/letsencrypt:/pebble:/g' -e 's/: letsencrypt/: pebble/g' -e 's,DirectoryURL: https://acme-v02.api.letsencrypt.org/directory,DirectoryURL: https://acmepebble.example:14000/dir,' -e 's/Submissions:$/Submissions:\n\t\t\tEnabledOnHTTPS: true/' -e 's/IMAPS:$/IMAPS:\n\t\t\tEnabledOnHTTPS: true/' config/mox.conf
+cat <<EOF >>config/mox.conf
+
+TLS:
+	CA:
+		CertFiles:
+                        # So certificates from moxmail2 are trusted, and pebble's certificate is trusted.
+			- /integration/tls/ca.pem
+EOF
+
+CURL_CA_BUNDLE=/integration/tls/ca.pem curl -o /integration/tmp-pebble-ca.pem https://acmepebble.example:15000/roots/0
+
+mox -checkconsistency serve &
+while true; do
+	if test -e data/ctl; then
+		echo -n accountpass1234 | mox setaccountpassword moxtest1
+		break
+	fi
+	sleep 0.1
+done
+wait