* bus/config-parser.c (struct BusConfigParser): Added
included_files field.
(seen_include): Added. Checks whether or not a file has already
been included by any parent BusConfigParser.
(bus_config_parser_new): Copy the parent's included_files.
(include_file): Track which files have been included, and fail on
circular inclusions.
(process_test_valid_subdir): Changed printf to report if we are
testing valid or invalid conf files.
(all_are_equiv): Changed printf to be a bit clearer about
what we are actually doing.
(bus_config_parser_test): Test invalid configuration files.
