Draft: polkitd: add logging for successfully loaded rules

Log each successful load operation of a rule file including the name of the loaded file. Getting the list of active rule files from the logs is an important information in a support case and should not need additional tracing/debugging.

Built and tested on top of 4b7a5c35. The info is logged during polkitd startup and with changes of rule files, also in case '--no-debug' is set. It might be a good idea to add a log_level to the argument list of polkit_backend_authority_log() but this would affect >60 callers and is out of scope of this patch.