Factor out unexpected handshake message handling code in the legacy stack.

Factor out unexpected handshake message handling code in the legacy stack.

The TLS record layer has to be able to handle unexpected handshake messages
that result when it has been asked to read application data. The way that
this is currently done in the legacy stack is a layering violation - the
record layer knows about DTLS/TLS handshake messages, parsing them and then
deciding what action to take. This is further complicated by the need to
handle handshake message fragments.

For now, factor this code out with minimal changes - since it is a layering
violation we have to retain separate code for DTLS and TLS.

ok beck@ inoguchi@ tb@