Allow "ECDHE" as a synonym of "EECDH" when specifiying ciphers

The standard terminology in https://tools.ietf.org/html/rfc4492 is
ECDHE.  "openssl ciphers" outputs ECDHE.  But users of the library
currently cannot specify ECDHE, they must specify EECDH.

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