Fix two more stupid bugs in 3des-ctr:

 - We were using the first word of each block of keystream block twice and the
   second not at all.
 - We were incrementing the high-order word of the counter after every block
   rather than the low-order one.

With those fixed, our 3des-ctr implementation interoperates with the one in
Moussh.  Thanks to der Mouse for his help with the testing.

3des-ctr is now enabled by default.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5699 cda61777-01e9-0310-a592-d414129be87e