New option no-ssl3-method which removes SSLv3_*method

When no-ssl3 is set only make SSLv3 disabled by default. Retain -ssl3
options for s_client/s_server/ssltest.

When no-ssl3-method is set SSLv3_*method() is removed and all -ssl3
options.

We should document this somewhere, e.g. wiki, FAQ or manual page.
Reviewed-by: Emilia Käsper <emilia@openssl.org>
Fix excert logic.

If no keyfile has been specified use the certificate file instead.

Fix typo: we need to check the chain is not NULL, not the chain file.
Reviewed-by: Matt Caswell <matt@openssl.org>

(cherry picked from commit 786370b1b09b919d9306f27336e13783e4fe3fd0)