A listening socket can be enabled with the sockopt
TCP_DEFER_ACCEPT. This informs the kernel to not call the user-land
accept() until real data has been written to the socket.
A new flag LEV_OPT_DEFERRED_ACCEPT has been introduced to
automatically set this option. Optionally
evutil_make_tcp_listen_socket_deferred() can be called manually.
(Tweaked slightly by nickm.)
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