Let afs_event() return int.

It is usually a critical error if an afs event handler returns an
error. Currently we only print a log message an continue in this
case. The callers of afs_event() which trigger the event have no way
to tell that something went wrong, since this function returns void.

By returning int instead of void the callers can abort in the error
case. Most of the callers are afs callbacks. These can propagate the
error code to the command handler process, which will translate the
error code into a string and send it to the client.  All callbacks
are changed in this way.