Fixing crash during TLS connection shutdown.

A crash was caused by an incorrect timer handler nxt_h1p_idle_timeout() if
SSL_shutdown() returned SSL_ERROR_WANT_READ/SSL_ERROR_WANT_WRITE.

The flag SSL_RECEIVED_SHUTDOWN is used to avoid getting SSL_ERROR_WANT_READ, so
the server won't wait for a close notification from a client.

For SSL_ERROR_WANT_WRITE, a correct timer handler is set up.

src/nxt_tls.h

@@ -69,6 +69,8 @@ struct nxt_tls_conf_s {
     char                          *ca_certificate;
 
     size_t                        buffer_size;
+
+    uint8_t                       no_wait_shutdown;  /* 1 bit */
 };