saner return codes for check_can{remove,rename}

These return codes are not currently used, but might be later.

IkiWiki/Plugin/remove.pm

@@ -71,6 +71,7 @@ sub check_canremove ($$$) {
 			}
 		}
 	});
+	return defined $canremove ? $canremove : 1;
 }
 
 sub formbuilder_setup (@) {

IkiWiki/Plugin/rename.pm

@@ -108,6 +108,7 @@ sub check_canrename ($$$$$$) {
 			}
 		}
 	});
+	return defined $canrename ? $canrename : 1;
 }
 
 sub rename_form ($$$) {