This commit adds support for reading new SSL configuration
options (ssl-ca, ssl-cert and ssl-key) if the [sst] section
with old options (tca, tcert and tkey) is missing in the config
file, even if not specified authentication mode via the ssl-mode
option. Before this change, new parameters were read only if the
ssl-mode option was present in the configuration file and it was
not equal to the 'DISABLED' value.

Also added diagnostics (information level) which warns the user
that due to the presence of the tca, tcert and/or tkey parameters
in the [sst] section, new SSL configuration options will be ignored
(if their values do not match the old ones).