use SSL_kDHE throughout instead of SSL_kEDH

DHE is the standard term used by the RFCs and by other TLS
implementations.  It's useful to have the internal variables use the
standard terminology.

This patch leaves a synonym SSL_kEDH in place, though, so that older
code can still be built against it, since that has been the
traditional API.  SSL_kEDH should probably be deprecated at some
point, though.