io: fix socket/fd port custodian registration

Register the port, not the file descriptor, especially since a TCP
connection can have ports that share a file descriptor. Also, I think
a weak reference in the custodian doesn't work as intended (visible
through finalization) if the file descriptor is referenced with a
callback that closes over the port.

racket/src/io/network/tcp-listen.rkt

@@ -4,6 +4,7 @@
          "../host/rktio.rkt"
          "../security/main.rkt"
          "../sandman/main.rkt"
+         "../format/main.rkt"
          "port-number.rkt"
          "address.rkt"
          "error.rkt")

racket/src/io/port/fd-port.rkt

@@ -432,9 +432,9 @@
 
 (define (register-fd-close custodian fd fd-refcount flush-handle port)
   (unsafe-custodian-register custodian
-                             fd
+                             port
                              ;; in atomic mode
-                             (lambda (fd)
+                             (lambda (port)
                                (when flush-handle
                                  (plumber-flush-handle-remove! flush-handle))
                                (if (input-port? port)