chasquid: Detect email loops

This patch implements some measures against email loops, such as keeping
a limit on the lenght of an address, and rejecting email that has too
many Received headers.

It's not perfect (a server could be actively removing Received headers),
but it should cover the normal accidents and misconfigurations.

test/t-09-loop/run.sh

@@ -0,0 +1,43 @@
+#!/bin/bash
+
+set -e
+. $(dirname ${0})/../util/lib.sh
+
+init
+
+rm -rf .data-A .data-B .mail
+
+# Two servers:
+# A - listens on :1025, hosts srv-A
+# B - listens on :2015, hosts srv-B
+#
+# We cause the following loop:
+#   userA -> aliasB -> aliasA -> aliasB -> ...
+
+CONFDIR=A generate_certs_for srv-A
+CONFDIR=A add_user srv-A userA userA
+
+CONFDIR=B generate_certs_for srv-B
+
+mkdir -p .logs-A .logs-B
+
+chasquid -v=2 --log_dir=.logs-A --config_dir=A \
+	--testing__outgoing_smtp_port=2025 &
+chasquid -v=2 --log_dir=.logs-B --config_dir=B \
+	--testing__outgoing_smtp_port=1025 &
+
+wait_until_ready 1025
+wait_until_ready 2025
+
+run_msmtp aliasB@srv-B < content
+
+# Wait until one of them has noticed and stopped the loop.
+while sleep 0.1; do
+	wget -q -O .data-A/vars http://localhost:1099/debug/vars
+	wget -q -O .data-B/vars http://localhost:2099/debug/vars
+	if grep -q '"chasquid/smtpIn/loopsDetected": 1,' .data-?/vars; then
+		break
+	fi
+done
+
+success