Don't fail the connection in SSLv3 if server selects ECDHE

Don't fail the connection in SSLv3 if server selects ECDHE

ECDHE is not properly defined for SSLv3. Commit fe55c4a2 prevented ECDHE
from being selected in that protocol. However, historically, servers do
still select ECDHE anyway so that commit causes interoperability problems.
Clients that previously worked when talking to an SSLv3 server could now
fail.

This commit introduces an exception which enables a client to continue in
SSLv3 if the server selected ECDHE.

[extended tests]

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3204)