Attach the accept "client" socket to .listener member of the socket

Attach the accept "client" socket to .listener member of the socket

When accepting a TCP connection in the higher layers (tlsstream,
streamdns, and http) attach to the socket the connection was accepted
on, and use this socket instead of the parent listening socket.

This has an advantage - accessing the sock->listener now doesn't break
the thread boundaries, so we can properly check whether the socket is
being closed without requiring .closing member to be atomic_bool.