Suppress logging output

[ulfllorenz]

We use dirsync in a Python script that is again a small part of the build preparation. In the absense of errors, this preparation step should be rather quiet and generates about 6 lines of output. Now that we use dirsync internally, it adds another 3 that attract much more developer attention than is warranted.

In principle, we could set the logger and discard all messages, but then we would not get warnings, which are important to us.

Hence, we request a "quiet" option that reduces output to the minimum, except for warnings and errors.