Fix SSL 2.0 rollback checking: The previous implementation of the

test was never triggered due to an off-by-one error.

In s23_clnt.c, don't use special rollback-attack detection padding
(RSA_SSLV23_PADDING) if SSL 2.0 is the only protocol enabled in the
client; similarly, in s23_srvr.c, don't do the rollback check if
SSL 2.0 is the only protocol enabled in the server.