courier: Let the users configure the mail delivery agent

This patch adds configuration options for the MDA binary and command line
arguments, and changes the (soon to be renamed) procmail courier to make use
of them.

chasquid.go

@@ -51,6 +51,9 @@ func main() {
 		go http.ListenAndServe(conf.MonitoringAddress, nil)
 	}
 
+	courier.MailDeliveryAgentBin = conf.MailDeliveryAgentBin
+	courier.MailDeliveryAgentArgs = conf.MailDeliveryAgentArgs
+
 	s := NewServer()
 	s.Hostname = conf.Hostname
 	s.MaxDataSize = conf.MaxDataSizeMb * 1024 * 1024