lftp: expected stdio messages in `LANG='C'`; other text fixes

* 'ERR' ->'ER'
* fix stderr skip: reveresd info
* cleanup, update comments

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

Signed-off-by: Oleg Verych <olecom@gmail.com>
history: original 2011-11-12 picture of transportation schema

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