Restore historical SSL_get_servername() behavior

Restore historical SSL_get_servername() behavior

Commit 1c4aa31d79821dee9be98e915159d52cc30d8403 modified the state machine
to clean up stale ext.hostname values from SSL objects in the case when
SNI was not negotiated for the current handshake.  This is natural from
the TLS perspective, since this information is an extension that the client
offered but we ignored, and since we ignored it we do not need to keep it
around for anything else.

However, as documented in https://github.com/openssl/openssl/issues/7014 ,
there appear to be some deployed code that relies on retrieving such an
ignored SNI value from the client, after the handshake has completed.
Because the 1.1.1 release is on a stable branch and should preserve the
published ABI, restore the historical behavior by retaining the ext.hostname
value sent by the client, in the SSL structure, for subsequent retrieval.

[extended tests]

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7115)