Let afs callbacks return an error code.

Let afs callbacks return an error code.

It was a design mistake that callbacks have no way to tell whether
they were successful. This commit changes the callback_function
typedef so that callbacks return int instead of void.

Naturally, every callback must be adjusted accordingly. Doing so would
make the patch a bit large, so as a first step we make all callbacks
(except path_brother_callback() and hash_sister_callback() which
are special) return zero. The return value is ignored at the moment,
so the changes of this commit have no effect yet.