Fix SRP authentication ciphersuites.

Fix SRP authentication ciphersuites.

The addition of SRP authentication needs to be checked in various places
to work properly. Specifically:

A certificate is not sent.
A certificate request must not be sent.
Server key exchange message must not contain a signature.
If appropriate SRP authentication ciphersuites should be chosen.
Reviewed-by: Matt Caswell <matt@openssl.org>