Lingering close changed to handle NGX_AGAIN.

Lingering close changed to handle NGX_AGAIN.

The "do { c->recv() } while (c->read->ready)" form used in the
ngx_http_lingering_close_handler() is not really correct, as for
example with SSL c->read->ready may be still set when returning NGX_AGAIN
due to SSL_ERROR_WANT_WRITE.  Therefore the above might be an infinite loop.

This doesn't really matter in lingering close, as we shutdown write side
of the socket anyway and also disable renegotiation (and even without shutdown
and with renegotiation it requires using very large certificate chain and
tuning socket buffers to trigger SSL_ERROR_WANT_WRITE).  But for the sake of
correctness added an NGX_AGAIN check.