patch for triggering SFTP process_write

[valerio balbi]

>Peter wrote:
>I would suggest using a more general approach, such as inotify in
>Linux or the equivalent in your system.

Thank's for the suggestion! I don't know inotify before...
After a first overview to the documentation I confirm my opinion.

It seems easier indicate the script to trigger on write directly into
sftp-server.
It seems strategic to notifiy the error to the sender directly into
the sftp session.

NOK if put or trigger fails
OK only if put and trigger ok

Valerio