Previously, we would not check the content of the script directory.
This meant, that "/etc/NetworkManager/dispatcher.d" almost always
contained something, namely the "pre-up.d" and "pre-down.d" directories.

Improve that by searching the directories for at least one
executable file.

Also, debug log the detected state of the dispatcher directories.

Signed-off-by: Thomas Haller <thaller@redhat.com>