rename variables, struct fields and functions to include an "x" when they can panic for handling errors

and document the convention in develop.txt.
spurred by running errcheck again (it has been a while). it still has too many
false to enable by default.

main.go

@@ -1661,11 +1661,11 @@ new mail deliveries.
 	}
 	mustLoadConfig()
 
-	ctl := xctl()
-	ctl.xwrite("stop")
+	xctl := xctl()
+	xctl.xwrite("stop")
 	// Read will hang until remote has shut down.
 	buf := make([]byte, 128)
-	n, err := ctl.conn.Read(buf)
+	n, err := xctl.conn.Read(buf)
 	if err == nil {
 		log.Fatalf("expected eof after graceful shutdown, got data %q", buf[:n])
 	} else if err != io.EOF {