Dedup dtls1_dispatch_alert()/ssl3_dispatch_alert().

Dedup dtls1_dispatch_alert()/ssl3_dispatch_alert().

The code for dtls1_dispatch_alert() and ssl3_dispatch_alert() is largely
identical - with a bit of reshuffling we can use ssl3_dispatch_alert() for
both protocols and remove the ssl_dispatch_alert function pointer.

ok inoguchi@ tb@