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/msmtprc

@@ -0,0 +1,28 @@
+account default
+
+host testserver
+port 1587
+
+tls on
+tls_trust_file config/certs/testserver/fullchain.pem
+
+from user@testserver
+
+auth on
+user user@testserver
+password secretpassword
+
+account smtpport : default
+port 1025
+
+account subm_tls : default
+port 1465
+tls_starttls off
+
+account baduser : default
+user unknownuser@testserver
+password secretpassword
+
+account badpasswd : default
+user user@testserver
+password badsecretpassword