FX.94 Material flow Real mode: connect again after the link ended on PLC exceptions

When a Real link ended after several exception answers, Studio's socket stayed open (only the page stopped), so pressing Connect again was refused with "already connected". RealLink.connect now closes the old socket first. Found by tools/plc-sim/test.mjs against a software Modbus slave. Also plc.rs: the below-1024 port rule is a plain function with a test.