Introduce xbps_plist_{array,dictionary}_from_file().

Those are a wrapper around xbps_{array,dictionary}_internalize_from_zfile()
that prints a debugging msg when the plist file cannot be internalized.

Update xbps to use these wrappers.

bin/xbps-dgraph/main.c

@@ -598,7 +598,7 @@ main(int argc, char **argv)
 	if (conf_file == NULL)
 		conf_file = _DGRAPH_CFFILE;
 
-	confd = xbps_dictionary_internalize_from_zfile(conf_file);
+	confd = xbps_plist_dictionary_from_file(&xh, conf_file);
 	if (confd == NULL) {
 		if (errno != ENOENT)
 			die("cannot read conf file `%s'", conf_file);