Stream: using ngx_stream_ssl_srv_conf_t *sscf naming convention.

Stream: using ngx_stream_ssl_srv_conf_t *sscf naming convention.

Originally, the stream module was developed based on the mail module,
following the existing style.  Then it was diverged to closely follow
the http module development.  This change updates style to use sscf
naming convention troughout the stream module, which matches the http
module code style.  No functional changes.