backup recommendations

1. Implicit usage of .cs-backup-config from current folder is unsafe. I recommend to use it only if -f option is specified or to use it from /etc folder.
2. -f option without file name give warning but continue backup. Warning is on the top of a lot of messages and is difficult to be found. I recommend raise error and not continue backup.
3. -f option with non-existing file doesn't raise warning or error. I recommend raise error and not continue backup.

4. backup -spoll option doesn't work for now.