Implement basic IDNA support

This patch implements the first steps of support for IDNA (Internationalized
Domain Names for Applications).

Internally, we maintain the byte-agnostic representation, including
configuration.

We support receiving IDNA mail over SMTP, which we convert to unicode for
internal handling.

Local deliveries are still kept agnostic.

For sending over SMTP, we use IDNA for DNS resolution, but there are some
corner cases pending in the SMTP courier that are tied with SMTPUTF8 and will
be fixed in subsequent patches.

test/t-06-idna/run.sh

@@ -0,0 +1,48 @@
+#!/bin/bash
+
+set -e
+. $(dirname ${0})/../util/lib.sh
+
+init
+
+rm -rf .data-A .data-B .mail
+
+skip_if_python_is_too_old
+
+# Two servers:
+# A - listens on :1025, hosts srv-ñ
+# B - listens on :2015, hosts srv-ü
+
+CONFDIR=A generate_certs_for srv-ñ
+CONFDIR=A add_user srv-ñ ñangapirí antaño
+CONFDIR=A add_user nadaA nadaA nadaA
+
+CONFDIR=B generate_certs_for srv-ü
+CONFDIR=B add_user srv-ü pingüino velóz
+CONFDIR=B add_user nadaB nadaB nadaB
+
+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
+
+# Send from A to B.
+smtpc.py --server=localhost:1025 --user=nadaA@nadaA --password=nadaA \
+	< from_A_to_B
+
+wait_for_file .mail/pingüino@srv-ü
+mail_diff from_A_to_B .mail/pingüino@srv-ü
+
+# Send from B to A.
+smtpc.py --server=localhost:2025 --user=nadaB@nadaB --password=nadaB \
+	< from_B_to_A
+
+wait_for_file .mail/ñangapirí@srv-ñ
+mail_diff from_B_to_A .mail/ñangapirí@srv-ñ
+
+success