maillog: Support logging to stdout and stderr

This patch adds support for writing maillog to stdout and stderr, which
can be desirable in certain environments.

Thanks to Denys Vitali <denys@denv.it> who sent an alternative patch for
this functionality.

test/t-17-maillog/run.sh

@@ -0,0 +1,48 @@
+#!/bin/bash
+
+set -e
+. $(dirname ${0})/../util/lib.sh
+
+init
+
+mkdir -p .logs
+
+generate_certs_for testserver
+add_user user@testserver secretpassword
+add_user someone@testserver secretpassword
+
+function send_one() {
+	rm -f .logs/mail_log .logs/stdout .logs/stderr
+	envsubst < config/chasquid.conf.in > config/chasquid.conf
+
+	chasquid -v=2 --logfile=.logs/chasquid.log --config_dir=config \
+		> .logs/stdout 2> .logs/stderr &
+	wait_until_ready 1025
+
+	run_msmtp someone@testserver < content
+	wait_for_file .mail/someone@testserver
+	mail_diff content .mail/someone@testserver
+
+	pkill -s 0 chasquid
+	sleep 0.2
+}
+
+export MAIL_LOG_PATH="../.logs/mail_log"
+send_one
+if ! grep -q "from=user@testserver all done" .logs/mail_log; then
+	fail "entries not found in .logs/mail_log"
+fi
+
+export MAIL_LOG_PATH="<stdout>"
+send_one
+if ! grep -q "from=user@testserver all done" .logs/stdout; then
+	fail "entries not found in .logs/stdout"
+fi
+
+export MAIL_LOG_PATH="<stderr>"
+send_one
+if ! grep -q "from=user@testserver all done" .logs/stderr; then
+	fail "entries not found in .logs/stderr"
+fi
+
+success