Problem:    Some users don't want to diff with hidden buffers.
Solution:   Add the "hiddenoff" item to 'diffopt'. (Alisue, closes #2394)

Problem:    In diff mode the cursor may end up below the last line, resulting
            in an ml_get error.
Solution:   Check the line to be valid.

Problem:    Generated function prototypes still have __ARGS().
Solution:   Generate function prototypes without __ARGS().

Problem:    Cannot specify the buffer to use for "do" and "dp", making them
            useless for three-way diff.
Solution:   Use the count as the buffer number. (James McCoy)