[rfc] hide rs_buffers_t from public interface

I was thinking on the weekend about what could be done to make the
librsync interface a bit simpler.

It might be better to have the library do all input and output through
user callbacks, rather than exposing its buffering at all.  This is to
say that something like rs_job_drive would be the main interface, and
rs_buffers_t would be removed, or at least not public.  When the
library needs some input data, it calls the input callback.  Similarly
for output and for reading blocks from the basis file.

The application might not necessarily be able to provide all the data
when the library needs it.  The callback should have the option to
either indicate that it is blocked (return RS_BLOCKED) or to process
only part of the data.

In short the callbacks would work a lot like unix read/write/pread on
a nonblocking socket.

Attached is a sample of how it might work.  I think I like it.

Any comments?
-- 
Martin

#include <unistd.h>
#include <librsync.h>

int main(void)
{
        const int in_fd = STDIN_FILENO, out_fd = STDOUT_FILENO;
        rs_job_t *job;
        rs_result result;

        rs_gensig_begin(&job,
                        rs_fd_read, &in_fd,
                        rs_fd_write, &out_fd);

        while ((result = rs_job_drive(job)) == RS_BLOCKED) {
                /* imagine a select() on stdin/stdout here, if we
                 * expected them to be nonblocking */
                ;
        }

        rs_job_free(job);

        return result;        
}

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