Restore '-keyform engine' support for s_client

This used to work in 1.0.2 but disappeared when the argument parsing was
revamped.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1639)

(cherry picked from commit a6972f34)