Ensure s_client and s_server work when read_ahead is set

Ensure s_client and s_server work when read_ahead is set

Previously s_client and s_server relied on using SSL_pending() which does
not take into account read_ahead. For read pipelining to work, read_ahead
gets set automatically. Therefore s_client and s_server have been
converted to use SSL_has_pending() instead.

Reviewed-by: Tim Hudson <tjh@openssl.org>