unix/modsocket: Set file descriptor to -1 on close.

After s.close(), s.fileno() now returns -1, matching CPython behavior.
Some code relies on this compatibility, as it allows checking whether
a socket is closed by testing its fileno() value. This change ensures
better interoperability with existing Python code and libraries.

Signed-off-by: Mike Wang <mikewang000000@gmail.com>

tests/extmod/socket_fileno.py

@@ -0,0 +1,17 @@
+# Test socket.fileno() functionality
+
+try:
+    import socket
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+if not hasattr(socket.socket, "fileno"):
+    print("SKIP")
+    raise SystemExit
+
+s = socket.socket()
+print(s.fileno() >= 0)
+
+s.close()
+print(s.fileno())  #  should print -1