Split hash_file into three steps, setup, add buf and final.

Split hash_file into three steps, setup, add buf and final.

Setup inits the context and adds the seed. The buf function simply adds
a block from the file to the hash. The final function calls MD4_Final()
to close the context and generate the hash.

This will help to remove the mmap in the sender and should result in a
more atomic view of the file since hash_file() is now called together
with the other hash_functions.

OK deraadt@ tb@