(unassign_path): New define for unassign_string.
(parameters): Use assign_path and unassign_path for all path
	parameters, to make them easier to find.  Use default values
	from paths.h instead of hardcoding the defaults in the file.
	A lot of values have changed; see the summary in the NEWS
	file.
(CONFIG_FILE): Removed.  This is now a define in paths.h.
(compiled_config_file): New constant.
(get_default_config_file_name): Use compiled_config_file instead
	of CONFIG_FILE.
(free_default_config_file_name): Ditto.