Address feedback on SSLv2 ClientHello processing

Feedback on the previous SSLv2 ClientHello processing fix was that it
breaks layering by reading init_num in the record layer. It also does not
detect if there was a previous non-fatal warning.

This is an alternative approach that directly tracks in the record layer
whether this is the first record.

GitHub Issue #1298

Reviewed-by: Tim Hudson <tjh@openssl.org>