server: use cross platform socket close
server: use cross platform socket close

`mk_server_listen_exit` previously used `close` to close the server file
descriptor. On Windows it needs to be `closesocket`. This PR switches
that function to use the cross-platform `mk_event_closesocket`.

Signed-off-by: braydonk <1c5258bd2a28a7131a97dab38e807a4145719a11@google.com>

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