PROCEDURE DoMail;
   EXTERNAL;
{*USER*
 
Process Network Mailbox input for AMIRTR.  Network data is common to all
active network links. Included are connect requests, interrupt messages,
and disconnect request/notification.
 
The mailbox is flushed each time this routine is called (except on net
abort, when we just quit and let DECNET clean up).
 
The Mailbox LUN is known from Net_Mbx_LUN.
 }