Flush stderr or stdout when logging

[ikus060]

Changes done and why

I have this scenario where output of rdiff-backup is scramble because, it seams, the stdout and stderr are printed to the console in different order. To get this fixed, I recommand to flush stderr and/or stdout on every log. This also push the lines to the console in real-time.

Self-Checklist

• [-] changes to the code have been reflected in the documentation: N/A
• [-] changes to the code have been covered by new/modified tests: N/A
• [ ] commit contains a description of changes relevant to users prefixed by DOC:, FIX:, NEW: and/or CHG: I'm open to recommendation.