Allow "DHE" and "kDHE" as synonyms of "EDH" and "kEDH" when specifiying ciphers

The standard terminology in https://tools.ietf.org/html/rfc5426 is
"DHE".  "openssl ciphers" outputs "DHE" (for the most part).  But
users of the library currently cannot specify "DHE", they must
currently specify "EDH".

This change allows users to specify the common term in cipher suite
strings without breaking backward compatibility.