lftpd.sh: stderr filter not app-errors like "no files found"

Signed-off-by: Oleg Verych <olecom@gmail.com>
app transportation API `app.lftp`: .on(event) .send(data)

initial implementation, config
stop/start fixes in `sh.on('close')`

Signed-off-by: Oleg Verych <olecom@gmail.com>
tl;tr comments with description of logic and `lftp` commands

Signed-off-by: Oleg Verych <olecom@gmail.com>