Improved AES-CFB performance using AES-NI insn.

There is also a new regression test which tests the bulk
encryption methods we have for a few ciphers (namely AES).  A bug in
them could have slipped through because we only did encrypt-decrypt
tests but didn't compared them to fixed vectors.

Benchmarks using gcc 4.4 show a 7 fold speed improvement for CFB
encryption and 14 for decryption.  This is a bit strange; someone
should check the code to see why we have this difference.

Without AESNI (undef USE_AESNI in rijndael.c):

$ ./benchmark --cipher-repetitions 100 --alignment 16 cipher aes aes256
Running each test 100 times.
                ECB/Stream         CBC             CFB             OFB             CTR
             --------------- --------------- --------------- --------------- ---------------
AES           1370ms  1430ms  1140ms  1190ms  1120ms  1130ms  1520ms  1540ms  1780ms  1770ms
AES256        1780ms  1850ms  1530ms  1610ms  1540ms  1530ms  1930ms  1960ms  2180ms  2180ms

With AESNI:

$ ./benchmark --cipher-repetitions 100 --alignment 16 cipher aes aes256
Running each test 100 times.
                ECB/Stream         CBC             CFB             OFB             CTR
             --------------- --------------- --------------- --------------- ---------------
AES             80ms   100ms   240ms   220ms   140ms    70ms   300ms   290ms   490ms   510ms
AES256         130ms   130ms   290ms   270ms   200ms   100ms   340ms   340ms   470ms   470ms

$ ./benchmark --cipher-repetitions 100 --alignment 0 cipher aes aes256
Running each test 100 times.
                ECB/Stream         CBC             CFB             OFB             CTR
             --------------- --------------- --------------- --------------- ---------------
AES             80ms    90ms   240ms   230ms   150ms    80ms   290ms   300ms   500ms   530ms
AES256         130ms   130ms   290ms   260ms   190ms   110ms   340ms   340ms   470ms   490ms

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