Split out DHE CKE construction into a separate function
Split out DHE CKE construction into a separate function

Continuing previous commit to break up the
tls_construct_client_key_exchange() function. This splits out the ECDHE
code.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl