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/util/smtpc.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+#
+# Simple SMTP client for testing purposes.
+
+import argparse
+import email.parser
+import email.policy
+import smtplib
+import sys
+
+ap = argparse.ArgumentParser()
+ap.add_argument("--server", help="SMTP server to connect to")
+ap.add_argument("--user", help="Username to use in SMTP AUTH")
+ap.add_argument("--password", help="Password to use in SMTP AUTH")
+args = ap.parse_args()
+
+# Parse the email using the "default" policy, which is not really the default.
+# If unspecified, compat32 is used, which does not support UTF8.
+msg = email.parser.Parser(policy=email.policy.default).parse(sys.stdin)
+
+s = smtplib.SMTP(args.server)
+s.starttls()
+s.login(args.user, args.password)
+
+# Note this does NOT support non-ascii message payloads transparently (headers
+# are ok).
+s.send_message(msg)
+s.quit()
+
+