Allow ECDHE and DHE as forward-compatible aliases for EECDH and EDH

see RT #3203

Future versions of OpenSSL use the canonical terms "ECDHE" and "DHE"
as configuration strings and compilation constants.  This patch
introduces aliases so that the stable 1.0.2 branch can be
forward-compatible with code and configuration scripts that use the
normalized terms, while avoiding changing any library output for
stable users.

Signed-off-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Matt Caswell <matt@openssl.org>