gsocket: Fix connected state if shutting down in two steps

The value of g_socket_is_connected() gets stuck high if the GSocket is
shut down in two steps:
   g_socket_shutdown (socket, TRUE, FALSE, NULL);
   g_socket_shutdown (socket, FALSE, TRUE, NULL);
rather than one:
   g_socket_shutdown (socket, TRUE, TRUE, NULL);

Fix that by tracking the connected status for the read half and the
write half of the connection separately.

https://bugzilla.gnome.org/show_bug.cgi?id=697907