Normalize domains

We should ignore the domains' case, and treat them uniformly, specially when it
comes to local domains.

This patch extends the existing normalization (IDNA, keeping domains as
UTF8 internally) to include case conversion and NFC form for
consistency.

test/t-07-smtputf8/content

@@ -1,5 +1,5 @@
-From: ñandú@ñoños
-To: ñangapirí@ñoños
+From: ñandú@ñoÑos
+To: Ñangapirí@Ñoños
 Subject: Arañando el test
 
 Crece desde el test el futuro

test/t-07-smtputf8/run.sh

@@ -1,5 +1,9 @@
 #!/bin/bash
 
+# Test UTF8 support, including usernames and domains.
+# Also test normalization: the destinations will have non-matching
+# capitalizations.
+
 set -e
 . $(dirname ${0})/../util/lib.sh
 
@@ -8,7 +12,10 @@ init
 skip_if_python_is_too_old
 
 generate_certs_for ñoños
-add_user ñoños ñangapirí antaño
+
+# Intentionally have a config directory for upper case; this should be
+# normalized to lowercase internally (and match the cert accordingly).
+add_user ñoñOS ñangapirí antaño
 
 # Python doesn't support UTF8 for auth, use an ascii user and domain.
 add_user nada nada nada
@@ -17,9 +24,12 @@ mkdir -p .logs
 chasquid -v=2 --log_dir=.logs --config_dir=config &
 wait_until_ready 1025
 
+# The envelope from and to are taken from the content, and use a mix of upper
+# and lower case.
 smtpc.py --server=localhost:1025 --user=nada@nada --password=nada \
 	< content
 
+# The MDA should see the normalized users and domains, in lower case.
 wait_for_file .mail/ñangapirí@ñoños
 mail_diff content .mail/ñangapirí@ñoños