httpd_config::close_fn field

Custom session closing callback. Called when a session is deleted, before freeing user and transport contexts and before closing the socket. This is a place for custom de-init code common to all sockets. The server will only close the socket if no custom session closing callback is set. If a custom callback is used, `close(sockfd)` should be called in here for most cases. Set the user or transport context to NULL if it was freed here, so the server does not try to free it again. This function is run for all terminated sessions, including sessions where the socket was closed by the network stack - that is, the file descriptor may not be valid anymore.