Add driusan/dkim integration example and tests

This patch adds DKIM signing using https://github.com/driusan/dkim tools
to the example hook.

It also adds an optional integration test to exercise signing and
verification, and corresponding documentation.

test/t-15-driusan_dkim/run.sh

@@ -0,0 +1,51 @@
+#!/bin/bash
+#
+# Test integration with driusan's DKIM tools.
+# https://github.com/driusan/dkim
+
+set -e
+. $(dirname ${0})/../util/lib.sh
+
+init
+
+for binary in dkimsign dkimverify dkimkeygen; do
+	if ! which $binary > /dev/null; then
+		skip "$binary binary not found"
+		exit 0
+	fi
+done
+
+generate_certs_for testserver
+( mkdir -p .dkimcerts; cd .dkimcerts; dkimkeygen )
+
+add_user user@testserver secretpassword
+add_user someone@testserver secretpassword
+
+mkdir -p .logs
+chasquid -v=2 --logfile=.logs/chasquid.log --config_dir=config &
+wait_until_ready 1025
+
+# Authenticated: user@testserver -> someone@testserver
+# Should be signed.
+run_msmtp someone@testserver < content
+wait_for_file .mail/someone@testserver
+mail_diff content .mail/someone@testserver
+grep -q "DKIM-Signature:" .mail/someone@testserver
+
+# Verify the signature manually, just in case.
+dkimverify -txt .dkimcerts/dns.txt < .mail/someone@testserver
+
+# Save the signed mail so we can verify it later.
+# Drop the first line ("From blah") so it can be used as email contents.
+tail -n +2 .mail/someone@testserver > .signed_content
+
+# Not authenticated: someone@testserver -> someone@testserver
+smtpc.py --server=localhost:1025 < .signed_content
+
+# Check that the signature fails on modified content.
+echo "Added content, invalid and not signed" >> .signed_content
+if smtpc.py --server=localhost:1025 < .signed_content 2> /dev/null; then
+	fail "DKIM verification succeeded on modified content"
+fi
+
+success