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/lib.sh

@@ -39,8 +39,11 @@ function chasquid() {
 }
 
 function add_user() {
+	CONFDIR="${CONFDIR:-config}"
+	mkdir -p "${CONFDIR}/domains/${1}/"
 	go run ${TBASE}/../../cmd/chasquid-util/chasquid-util.go \
-		adduser "config/domains/${1}/users" "${2}" --password "${3}" \
+		adduser "${CONFDIR}/domains/${1}/users" "${2}" \
+		--password "${3}" \
 		>> .add_user_logs
 }
 
@@ -52,6 +55,10 @@ function run_msmtp() {
 		msmtp -C msmtprc "$@"
 }
 
+function smtpc.py() {
+	${UTILDIR}/smtpc.py "$@"
+}
+
 function mail_diff() {
 	${UTILDIR}/mail_diff "$@"
 }
@@ -82,9 +89,21 @@ function wait_for_file() {
 
 # Generate certs for the given hostname.
 function generate_certs_for() {
-	mkdir -p config/certs/${1}/
+	CONFDIR="${CONFDIR:-config}"
+	mkdir -p ${CONFDIR}/certs/${1}/
 	(
-		cd config/certs/${1}
+		cd ${CONFDIR}/certs/${1}
 		generate_cert -ca -duration=1h -host=${1}
 	)
 }
+
+# Check the Python version, and skip if it's too old.
+# This will check against the version required for smtpc.py.
+function skip_if_python_is_too_old() {
+	# We need Python >= 3.5 to be able to use SMTPUTF8.
+	check='import sys; sys.exit(0 if sys.version_info >= (3, 5) else 1)'
+	if ! python3 -c "${check}"; then
+		skip "python3 >= 3.5 not available"
+		exit 0
+	fi
+}