s_client: use the full buffer for reads

s_client: use the full buffer for reads

Use full allocated buffer for reads to not call into switch() over and
over; also increase the size of the buffer to 16 kiB (max for TLS
records). The server side already is using 16 kiB buffers.

Signed-off-by: Hubert Kario <hkario@redhat.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/24688)