Take advantage of PUT_32BIT_MSB_FIRST when constructing sequence numbers
to MAC.


git-svn-id: svn://svn.tartarus.org/sgt/putty@9758 cda61777-01e9-0310-a592-d414129be87e
Add support for HMAC-SHA-256 as an SSH-2 MAC algorithm ("hmac-sha2-256") 
as specified in RFC 6668.  This is not so much because I think it's 
necessary, but because scrypt uses HMAC-SHA-256 and once we've got it we 
may as well use it.

Code very closely derived from the HMAC-SHA-1 code.

Tested against OpenSSH 5.9p1 Debian-5ubuntu1.


git-svn-id: svn://svn.tartarus.org/sgt/putty@9759 cda61777-01e9-0310-a592-d414129be87e