remodelled SFTP code pushed

Hi friends,

I've pushed my most recent SFTP changes as well now, so please have a go at it
and see if it still works for you and perhaps even check how fast/slow it
works for use cases. I'm sure I've made mistakes and that there are further
tweaks to be done.

The fact that the code does some assumptions on the data provided in the
function calls I fear that it might break some applications and I'm
considering adding a way to disable this "write ahead" concept.

I intend to introduce the same concept for SFTP reading next.

-- 
  / daniel.haxx.se
_______________________________________________
libssh2-devel http://cool.haxx.se/cgi-bin/mailman/listinfo/libssh2-devel