hooks: Add dkimpy support

This patch adds support in the default hook for using dkimpy for DKIM
signing.

Unfortunately, dkimpy binaries have the same name as driusan/dkim's, so
we need to use --help to disambiguate. It's not pretty but it should
work, and is quite self contained.

Also, for the integration tests, we still need driusan/dkim because
dkimpy lacks the features needed. Specifically, dkimpy's dkimverify
can't be made to use custom DNS, or override the TXT values in any way,
so we can't verify that the generated signature is reasonable.

Thanks to ne9z@github for suggesting this change and providing an
alternative patch in https://github.com/albertito/chasquid/pull/19.

etc/chasquid/hooks/post-data

@@ -7,7 +7,7 @@
 #  - spamc (from Spamassassin) to filter spam.
 #  - rspamc (from rspamd) or chasquid-rspamd to filter spam.
 #  - clamdscan (from ClamAV) to filter virus.
-#  - dkimsign (from driusan/dkim) to do DKIM signing.
+#  - dkimsign (from driusan/dkim or dkimpy) to do DKIM signing.
 #
 # If it exits with code 20, it will be considered a permanent error.
 # Otherwise, temporary.
@@ -79,7 +79,7 @@ if command -v clamdscan >/dev/null; then
         echo "X-Virus-Scanned: pass"
 fi
 
-# DKIM sign with https://github.com/driusan/dkim.
+# DKIM sign with either driusan/dkim or dkimpy.
 #
 # Do it only if all the following are true:
 #  - User has authenticated.
@@ -90,12 +90,34 @@ fi
 # Note this has not been thoroughly tested, so might need further adjustments.
 if [ "$AUTH_AS" != "" ] && command -v dkimsign >/dev/null; then
 	DOMAIN=$( echo "$MAIL_FROM" | cut -d '@' -f 2 )
+
 	if [ -f "domains/$DOMAIN/dkim_selector" ] \
-			&& [ -f "certs/$DOMAIN/dkim_privkey.pem" ]; then
-		dkimsign -n -hd \
-			-key "certs/$DOMAIN/dkim_privkey.pem" \
-			-s "$(cat "domains/$DOMAIN/dkim_selector")" \
-			-d "$DOMAIN" \
-			< "$TF"
+		&& [ -f "certs/$DOMAIN/dkim_privkey.pem" ];
+	then
+		# driusan/dkim and dkimpy both provide the same binary (dkimsign) but
+		# take different arguments, so we need to tell them apart.
+		# This is awful but it should work reasonably well.
+		if dkimsign --help 2>&1 | grep -q -- --identity; then
+			# dkimpy
+			dkimsign \
+				"$(cat "domains/$DOMAIN/dkim_selector")" \
+				"$DOMAIN" \
+				"certs/$DOMAIN/dkim_privkey.pem" \
+				< "$TF" > "$TF.dkimout"
+			# dkimpy doesn't provide a way to just show the new
+			# headers, so we have to compute the difference.
+			# ALSOCHANGE(test/t-19-dkimpy/config/hooks/post-data)
+			! diff --changed-group-format='%>' \
+				--unchanged-group-format='' \
+				"$TF" "$TF.dkimout"
+			rm "$TF.dkimout"
+		else
+			# driusan/dkim
+			dkimsign -n -hd \
+				-key "certs/$DOMAIN/dkim_privkey.pem" \
+				-s "$(cat "domains/$DOMAIN/dkim_selector")" \
+				-d "$DOMAIN" \
+				< "$TF"
+		fi
 	fi
 fi