openssl enc: Don't unbuffer stdin

 - unbuffer causes single-byte reads from stdin and poor performance

Fixes #3281
CLA: trivial

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3299)
(cherry picked from commit 65d62488)