New environment to disallow 2 iocs on the same computer

src/rsrv/caservertask.c

@@ -110,6 +110,13 @@ static void req_server (void *pParm)
     status = bind ( IOC_sock, (struct sockaddr *) &serverAddr, sizeof ( serverAddr ) );
 	if ( status < 0 ) {
 		if ( SOCKERRNO == SOCK_EADDRINUSE ) {
+            const char* multiple_iocs = getenv("EPICS_ALLOW_MULTIPLE_IOCS");
+            if (multiple_iocs && strcasecmp(multiple_iocs, "NO") == 0)
+            {
+                errlogPrintf ( "CAS: Default server port %d unavailable.\n" ,ca_server_port);
+                errlogPrintf ( "CAS: Probably already running an IOC on this computer.\n");
+                epicsExit(1);
+            }
 			/*
 			 * enable assignment of a default port
 			 * (so the getsockname() call below will