Introduce afs_max_size_handler_data and afs_max_size_handler().

Whenever the maximal size of a para buffer is reached, para_printf()
calls the specified max size handler. This handler takes, in addition
to the actual buffer and its size, a void * context pointer.  Currently
all commands simply pass a pointer to the underlying file descriptor
here since this is the only context the handler needs to know about.

However, this is going to change since with sideband connections the
handler needs to know also the sideband designator.

To meet this future need, this patch introduces struct
afs_max_size_handler_data, which consists of the file descriptor only
at the moment, as well as a simple wrapper for pass_buffer_as_shm().