check_mailq: adding nullmailer support

Adding nullmailer support to check_mailq submitted by Luca Corti

Closes: #740
Closes: #1189

THANKS.in

@@ -307,3 +307,4 @@ Oskar Liljeblad
 Geoff Oakham
 Tilmann Bubeck
 Stéphane Bortzmeyer
+Luca Corti