Make CBC decoding constant time.

This patch makes the decoding of SSLv3 and TLS CBC records constant
time. Without this, a timing side-channel can be used to build a padding
oracle and mount Vaudenay's attack.

This patch also disables the stitched AESNI+SHA mode pending a similar
fix to that code.

In order to be easy to backport, this change is implemented in ssl/,
rather than as a generic AEAD mode. In the future this should be changed
around so that HMAC isn't in ssl/, but crypto/ as FIPS expects.

ssl/ssl3.h

@@ -355,6 +355,10 @@ typedef struct ssl3_record_st
 /*r */	unsigned char *comp;    /* only used with decompression - malloc()ed */
 /*r */  unsigned long epoch;    /* epoch number, needed by DTLS1 */
 /*r */  unsigned char seq_num[8]; /* sequence number, needed by DTLS1 */
+/*rw*/	unsigned int orig_len;  /* How many bytes were available before padding
+				   was removed? This is used to implement the
+				   MAC check in constant time for CBC records.
+				 */
 	} SSL3_RECORD;
 
 typedef struct ssl3_buffer_st