Warn when the default path is overwritten * load-path.h (dir_info): New member, is_init, true if this directory was set during init. (dir_info::dir_info): Initialise is_init param. (load_path::do_clear): Pass the new elements to add after clearing. (load_path::clear): Pass dummy empty elements. (load_path::do_set, load_path::do_append, load_path::do_add): Pass new bool if adding or setting an init directory. * load-path.cc (load_path::do_initalize, load_path::do_append, load_path::do_add): Pass around extra is_init argument. (do_set): Pass an std::set of new directories to do_append. (do_clear): Check if the init directories are getting removed and warn if so.