scanner: add a way to prune the scan process of media directories.

* If a ".mediaignore" file is present in a directory, the scan will ignore all regular files but will go through subdirectories.
* If a ".mediaignoreall" file is present in a directory, the scan will skip this directory and all subdirectories.
* The ".mediaignore" or ".mediaignoreall" files removal will not trigger a directory scan.